The position of the motor in the mounting bracket may be causing the arms to come to rest at a slightly different position, if you look at the mounting bracket there is a little room to move it in order to center it.
The arms themselves can be pooped off and rotated one or two splines to keep it from traveling as far.
This will make them move further in their arc on the other direction but between those two adjustments you should be able to get them centered.
Ray
