I wondered about this too...the impressions are the number of times each ad is seen, so if you have 3 image banners on a page, and the page gets 1000 impressions, you would get 3000 adsense impressions.
But what if instead of 3 image banners, they serve up three 4 text ad units. Does each ad in the unit get an impression, or does each ad unit get an impression. That would be the difference between 12000 impressions and 3000 impressions.
But what if instead of 3 image banners, they serve up three 4 text ad units. Does each ad in the unit get an impression, or does each ad unit get an impression. That would be the difference between 12000 impressions and 3000 impressions.