da Rock

Here is a quick sketch to illustrate the problem with square hinge joints.

When the pushrod pulls the horn a given amount, the surface deflects (in the sketch) 22 degrees down.

When that same pushrod pushes the horn the same amount, the surface rotates off a different point and in this case the surface only deflects 10 degrees.

The difference in deflection is a function of how far apart the top and bottom of the joint is and how far from the hinge line the pushrod connects from that hinge line. So if the surfaces aren't very thick, there won't be a big deal. But if you're hinging an aileron on a decent sized model, you will see significant differential.