I actually noticed it and tied it to the very day I enabled AdSense. I have not run ads in years but I enabled them on the evening of May 19th, and suddenly took a big hit in pages indexed and in page views reported by Google Analytics. Oddly enough CF analytics remains constant.
This is a pretty new import from vBulletin. I installed Google Analytics on May 4 and you can see it start tracking well. Huge spike on May 11 which appears to be a bot scrape or a DDoS attack. Google actually shows a drop on May 11 which makes sense because I shut some stuff down. Google only tracks if the javascript gets loaded, CF tracks via traffic through their network so it doesn't matter if a page actually loads or not. It records the attempt.
AdSense (which is NOT linked to my Analytics account) enabled the evening of May 19 and page views completely tanked. On May 25th they actually show fewer page views than the number of posts I personally made. I made 11 public posts plus a few PMs that day, they show 9 total page views.
Note that the CF stats are Visits and someone usually has more than a single page view per visit, and the Google stats are page views. Before the 19th these were roughly lining up.
For today CF shows 27k page views. Google Analytics says 9k. Google AdSense says 7k. Nothing weird here though...