For the past several months, Google has had an issue with reporting on pageviews for the second ad unit on a page. The issue has become more noticeable now that the AdSense beta is in the hands of all publishers.
A WebmasterWorld thread brought this old issue to light for me. The publisher explained "only the first ad unit on a page triggers a page view. Thus, if you have two ad units on a single page (header and footer) and look at channel stats for the second ad unit, it will report zero (or nearly zero) PAGE impressions while the ad unit impressions, clicks, etc. are correct."
This can be deceiving for some publishers.
Google AdSense representative, Engineer Guillaume, replied to the issue in detail saying in a Google AdSense Help thread:
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.
Forum discussion at WebmasterWorld & Google AdSense Help.
