I am an newbie at all of this...but..when I had similar problems, one of my club members pointed out that I didn't have the servo's arms exactly equal. One was "forward" a bit and that servo had a different ratio of throw.

Another possible cause is if different holes were used (control arm or control horn) on either end.

The last thought would be whether or not the control horns were equally positioned directly over the hinge line(s). Hope something here helps. Smarter and more experienced people will probably pipe in with the real correct answer. Rich