Well there is always some difference between the servos and sometimes reversing (change sides) or switching arms will help, but one BIG issue I see is the servos you are using. IIRC that was the first digital servo Hitec came out with and they are notoriously bad at centering and difficult to program.

I'd start by replacing the servos and then get yourself a Hitec programmer (assuming you stay with Hitec) and set 'em where you want them.....