"The discrepancies in those numbers are quite consistent, over time."
So, how much time are you observing?
The simple fact that they are consistent is a pretty solid indication that these are, in fact, regularly scheduled crawlers and not humans. Human patterns would tend to vary considerably.
They will target the forums that are most active.
At least that's how I understand them to work.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.