I have seen this opening a lot and most people don't even know of this. So thought of making a topic about it.
When you're repping someone, whom you may have repped before, your reputation gets updated with the value you choose. For example you rep a guy whom you have repped before. Lets say that he has 214 and you have +10 him before. If you update the rep with +5, it won't increase the rep but decrease it.
This is because FK doesn't allow multiple rep from same user.
Hope this helps.