If you have more up than down aileron movement, that's actually helpful. You can get differential with your mechanical setup also if your radio doesn't have that feature. The differential keeps the plane from doing an uncommanded yaw when you apply ailerons, and it's part of every trim process for every plane. To get differential without using your radio, set the servo arms to point toward the front by one or two teeth then lengthen the push rod to get the aileron back to neutral. Because of the geometry change, you'll now have more up travel than down travel, even though your radio is commanding equal travel to the servo. It takes some experimenting to get it right, but the test is to fly the plane away from yourself and roll left and right quickly maybe 15 degrees each way. If the tail wags while you're doing it, you have the wrong amount of differential set up. If it holds steady, you have it right. If the tail moves the opposite direction of the roll, you need more differential. If it moves the same direction, you have too much. Once you get it right, you'll find that rolls get smoother as do you standard turns.