Hmm I'm a bit confused should not 1 pageview = 1 impression? My pageviews are 10,000 according to Urchin and Impressions are 2,000 according to google. I have adsense on all my pages. Same for my other site... its similar and its running yahoo.
Could it be that your are using SSIs on your pages? I think some stat programs count each include as it's own file. So if you have about 5 includes then that would make sense as to why you are getting 10,000 in your stats, but only 2000 in google (which is javascript -- client side) and therefore doesn't count includes as seperate pages. I could be completely wrong though