What causes Google Adsense to report less than half the pageviews of web server stats

[harleyx]

I have the luxury of being able to go back to 2004 for google adsense stats, as well as web server stats, so I have over 5 years of data to look at.

Anybody have any theories why Google Adsense shows less than half, (sometimes as little as 20%) of the pageviews reported by the web servers statistics?

I assume a small portion of this difference may be explained away with banned IP addresses or ranges tagged by Google's system to not to count because of foreign traffic, playing dirty pool, and so on, although the majority of my traffic has always come from the US/Canada, a bit from Japan and the UK, and a very small percentage from other places.

I assume they have very strict filters for search engine spiders and so on, which may catch some that webstats programs do not.

I assume they don't count public service ads as well, although they aught to have enough ad stock to keep ads running at least 80-90% of the time on a website of such a common topic.

I've always assumed they don't display legitimate pageview numbers to make their click-through rates and effective CPM look better. It's the only thing I could figure makes any sense. Although that's just my conspiracy theory, I've never heard a solid explanation to this gap in statistics.

I've perused the raw server stats before, and the webstat programs interpreting them (webalized and awstats) appear to display normal and reasonable counts.

Any ideas?

[Seraphim]

I've noticed the exact same thing, but sadly it's unlikely we'll ever know why this occurs. I personally always thought it might have something to do with multiple ad units, combined with shoddy tracking on Google's part. I wrote it off a long time ago as just one more odd clanking sound behind the iron curtain...

[David G]

I have spent years trying to understand the difference based on my (valid looking) awstats and weblizer data without success