I am not a computer geek, or even a novice, but as I understand it, when activity picks up, adsense has more bots scanning for content. So if you have say a dozen guys posting on any given day, you could have 400 "guests" reading (scanning) the posts, detecting key words, logging, etc,etc.
I moderate on the Contractor Talk Forum, the largest contractor based forum online, and this is typical: Currently Active Users: 9511 (112 members and 9399 guests)
And we have: Threads: 174,396, Posts: 2,658,675, Members: 126,571
Now, obviously, not all registered members are active. But the archives are very real. The forum is owned by a company that owns many, many forums, and is a hot property. You get this much activity and the ad revenue is impressive.