Mechanical differential can can be a good thing on certain types of airplanes. It normally may not provide as much diff as can be accomplished electronically but still very useful. Charlie P's chart is of course correct but still may be confusing to some so to the Original poster here it is applied to a single servo high wing airplane Notice the pushrods go past a line drawn between the output bushing and the two pushrod connections to aheve the desirable differential that causes the upward moving aileron to move further than the downward moving aileron.

Mike is also correct in that the mechanical differential can also be acheved even with a two servo setup simply by the fore and aft control horn positioning out on the aileron in relation to the hingeline. The most well known application of this was the anniversery addition of the Goldbergh Cub which acheved diff of almost fifteen degrees via control horn placement on purpose. I don,t know about sigs but anyway perhaps this photo can also help:

John