Sorry, guys. You were right and I got confused, here's why:
The walkthrough did in fact say level 13 was the maximum. It also mentioned, like you guys, that at higher levels the offhand is much stronger than the main hand.
HOWEVER, it seems that these values were changed for the DKS version of the game. In DKS, even at level 13, you only get 25% main hand AND 25% offhand, making them equal at the top level (which, if you think about it, makes sense... if your "main" hand became more effective than your offhand, then your "main" hand could no longer even be called that :p). And this is why I thought you could still raise them a little bit higher than 13 - because I was waiting for that level in which my offhand would surpass my main, but it never showed up since the skill was rebalanced in DKS.
Thanks for your help, sorry for the confusion.
