The new adsense interface has an option to show ad impressions by ad units. But these was some problem with this when i saw the reports. There are 2 ad units present on all the pages of my site. But one was showing around 50,000 impressions while the other ad unit which is also present on all the pages had just near zero impressions. And an CTR nearly 500%. I was shocked and though this is alarming and Adsense my think I am using illegal methods to get excess income.

adsense ad units wrong impressions

adsense ad units wrong impressions

While searching for i answer i stumble to this answer by Google employee which stated that the problem is from Google and its a know issue. The actual problem is that Adsense tracks the page impressions only for the first ad unit on the page all the other ad units on page are given zero page impressions. Below is what the Google employee has to say about this.

Google Employee
10/16/10
Hi,
We are aware of this problem. In a nutshell, page views are counted only for the first ad unit of the page.
It affects all publishers; it is not a temporary glitch but a question of design.
Unfortunately there seems to be no solution that shows correct CTR and RPM in the ad units report while keeping reports consistent with each other. We would be happy to get your input if you have ideas about how to improve the situation.
In the mean time, we recommend using ‘page unit impressions | queries’ column instead of ‘page views’. We are considering making it the default to reduce confusion.
If you are interested, below is a detailed explanation of the problem:
As you noticed, when there are multiple ad units on a page, only the first one gets the page view. Each page view is counted exactly once in each report, so the total of the ‘page views’ column is the sum of all ‘page views’ values in the report, and matches the other reports. (Let’s ignore the special case of custom channels.)
Reports where each page view has statistics for a single row in the report behave as expected: Days/Weeks/Months, Products, URL channels, Sites. For instance, a given page view / click belongs to a single day.
The problem is for reports where a single page view generates statistics for multiple rows: Ad units, Ad sizes, Custom channels, Ad types, Targeting types, Bid types. For instance, if a page contains ad units with different ad sizes / ad types / etc., then only the ad size / ad type / etc. of the first ad unit will get the page view. The other gets no page views, hence show invalid page views, page CTR, and page RPM.
If page views were counted for all ad units, not only the first one, then the total for the ‘page views’ column on an ad unit report would be wrong and inconsistent between reports. For instance, let’s assume that we have 10 page views for a page containing 3 ad units, all of the same size. The ‘ad units’ report would show 10 page views per ad unit = 30 page views total (wrong), hence an incorrect total page CTR and page RPM. All ad units have the same size, so the ‘ad sizes’ report should show 1 row with 10 page views, which would not match the ‘ad units’ report. Showing 1 row with 30 page views would be wrong too, because there were only 10 page views in reality.
As you can see, we are in a dead end: whatever we do, the ‘page views’ total is wrong and/or inconsistent across reports.
A last remark: a similar issue affects ad unit impressions | queries for ad-level reports such as Targeting types and Bid types. The individual ads in an ad unit can have different targeting types or bid types. However, it is nearly invisible because:
1) Targeting types and bid types are evenly distributed among the ads of a given ad unit, so the corresponding reports are statistically sound.
2) No report allows getting statistics for the ads by position in their ad unit.
Guillaume

Hi,
We are aware of this problem. In a nutshell, page views are counted only for the first ad unit of the page.It affects all publishers; it is not a temporary glitch but a question of design.
Unfortunately there seems to be no solution that shows correct CTR and RPM in the ad units report while keeping reports consistent with each other. We would be happy to get your input if you have ideas about how to improve the situation.
In the mean time, we recommend using ‘page unit impressions | queries’ column instead of ‘page views’. We are considering making it the default to reduce confusion.

If you are interested, below is a detailed explanation of the problem:

As you noticed, when there are multiple ad units on a page, only the first one gets the page view. Each page view is counted exactly once in each report, so the total of the ‘page views’ column is the sum of all ‘page views’ values in the report, and matches the other reports. (Let’s ignore the special case of custom channels.)
Reports where each page view has statistics for a single row in the report behave as expected: Days/Weeks/Months, Products, URL channels, Sites. For instance, a given page view / click belongs to a single day.
The problem is for reports where a single page view generates statistics for multiple rows: Ad units, Ad sizes, Custom channels, Ad types, Targeting types, Bid types. For instance, if a page contains ad units with different ad sizes / ad types / etc., then only the ad size / ad type / etc. of the first ad unit will get the page view. The other gets no page views, hence show invalid page views, page CTR, and page RPM.

If page views were counted for all ad units, not only the first one, then the total for the ‘page views’ column on an ad unit report would be wrong and inconsistent between reports. For instance, let’s assume that we have 10 page views for a page containing 3 ad units, all of the same size. The ‘ad units’ report would show 10 page views per ad unit = 30 page views total (wrong), hence an incorrect total page CTR and page RPM. All ad units have the same size, so the ‘ad sizes’ report should show 1 row with 10 page views, which would not match the ‘ad units’ report. Showing 1 row with 30 page views would be wrong too, because there were only 10 page views in reality.
As you can see, we are in a dead end: whatever we do, the ‘page views’ total is wrong and/or inconsistent across reports.

A last remark: a similar issue affects ad unit impressions | queries for ad-level reports such as Targeting types and Bid types. The individual ads in an ad unit can have different targeting types or bid types. However, it is nearly invisible because:1) Targeting types and bid types are evenly distributed among the ads of a given ad unit, so the corresponding reports are statistically sound.2) No report allows getting statistics for the ads by position in their ad unit.
Guillaume