I have seen these on the gun forums I belonged to, if it is similar at all, the "Reputation Rating" is based on more than just that "Rep" button...

It depends on several factors, Threads usually play a huge part in it, not only the amount that you post, but how many people read them, and respond to them...

Friend lists sometimes are tied to your rep also, how many people are on your friend lists....

The thanks button is probably tied into the overall rating also...

Basically most of these rep systems are a matrix function of your overall contribution to the forum....