I was wondering about that myself. I am probably getting about half to PSA's from what I can tell. My weblog shows about 1500 page views a day on my site. But google only shows 700-800 "impressions" a day. I assume that when google shows a PSA it does not count as in impression?
It could be because my site is pretty new. I've only started getting inbound links in the last few weeks, and google still shows me as a PR of 0.
Hey, thanks your right. I poked around my site a bit and found that most of the time the PSA's are once I log into PHPbb and get that session=28349028390 thing tacked onto my URL. I guess thats whats bugging adsense.
On the areas of my site that I did the PHP for, I use mod_rewrite, and they always seem to have adsense there.