Since I started with adsense about 8 months ago, I set up my ads on different channels, even where the ads are on the same page. So, I have one channel for "top" and one for "side". The only places that "top" and "side" are used is the same pages. Every day since the beginning, the number of impressions between the two has been about 5% off. I would expect that 2 ads on the same page would have the same number of impressions. Would that indicate that some of the ads are left blank by google for some percentage of impressions?