Here is a picture that shows what happens with the rigging that is very often used with smaller biplanes.

You wind up with the uniform movement completely screwed up, a real mess. When the servo moves the aileron it drives directly a certain amount, the paired one will move a very different amount. When the same servo moves it's aileron in the other direction, the paired will move a DIFFERENT amount than it did the other way!

What you wind up with are 4 ailerons moving, with the two that are being driven by the servos moving the same deflection as each other. No problem. But on one side of the airplane, the paired aileron moves a different amount than the one it's connected to. So you got one side somewhat screwed up a certain amount. Now, on the other side of the airplane, the paired aileron moves an entirely different amount than any of the other 3 ailerons.

It's really screwed up and hard to put into words.

But here's a picture of the side of the biplane that has the downgoing ailerons. Let's pretend the servos drive the lower aileron. And the servo on one side is driving down. The servo commands a 20degree deflection and what the rigging does is change that deflection to the paired aileron to 26degrees down. So on one side there is a 6degree difference in deflections. OK, that's just on that side of the airplane.

On the other side, that rigging design doesn't screw up the same when the ailerons are being deflected upwards. Over there, the servo also commands 20degrees and whichever aileron is directly driven will do it. But when that rigging design moves in the up direction, the amount of difference is different. You don't even get the same screwup side to side. On the up side you get 23degrees from the driven one.

Talk about lack of uniformity. You have two ailerons going 20degrees, a third aileron going 23, and the fourth one going 26.

OK, why is this? And what can you do to fix it.