I am one of those people who doesn’t often look into my AdSense site diagnostics tab, but there has been a bug in it for the past week or so that was first noticed in the SERoundtable forums.
If someone views your site in the Google cache, they still see ads. But it seems to trigger a robots.txt error in your site diagnostic tab, because it recognizes that the IP used by the cache is blocked by robots.txt. So in your account, it is showing the Google cache URL, since the bot recognizes that those Google cache IP URLs are disallowed by Google’s robots.txt file for each of those IPs. I was also surprised at all the cache page views on various sites.
I did talk to Google about the issue, and it is being fixed. But in the meantime, you can login to your AdSense account and see it in your site diagnostics tab.