Reputation power increases by the number of posts that you have, and also if a moderator likes one of your posts they can add to your reputation, or take away from it if they do not like the post.

As for why I have less, I am not really sure. It's either a bug in the software or misconfiguration... I have not been able to figure it out. Every time I reset the counts from the forum admin it resets myself and the other mods to 10. Weird!