Somewhere in the system you have the pushrods connecting to the servo or the control horn at an angle or the output arm or control horn is not at right angles to the hinge lines.

What you have is differential throw caused by the geometric misalignment of some or all of the parts from above. Correct the angles or compensate for one part with a deliberate offsetting of the other and you'll get what you want.

For example if the pushrod meets the control horn such that the travel axis to control horn line is not a proper right angle then you can compensate for that by offsetting the servo arm and retrimming the pushrod length. If this doesn't make sense it would help greatly if you can provide detailed closeup photos of the control horn and pushrod at the rudder and at the servo. Top down or bottom up to show the angles would be best. From those we could hellp you more.