I was thinking there could be a time frame involved to make sure people just dont go nuts repping one another just for reps sake.(like a safety feature/abuse feature) And I was also thinking that when someone is getting reps quickly that Admin sees this and configures accordingly. Probably a super long shot on that one but hey ya neva know!
With regards to some peoples post numbers and join date it could be that some have had negative reps that held them for a while. And some people have like less than 30 posts yet seem to have 3 reps. They seem to be people that were on here long ago in which case they could have made some helpful or good posts that people are still repping them on or you get one for every year.
This next idea is really whacky but here goes anyway-
Maybe you can only get a certain amount of credited reps from one thread.

I would like to think this could be a possibility, however I have not read anything about that in any explination regarding the infrastucture of the rep system.
From what I have read some configurations require 50 for the second rep and in some cases way,
way,
WAY more.