The perch holes in the axle are inclined like the kingpin holes. Meaning if you put te perches in the holes they would be leaning toward the center of the axle.

The angles built into the batwings are to correct this inclination so that the 4-bars/haripins are vertical and not layed over. If you slip the batwing over the axle and look down on it from the top you will quickly see which was is right or wrong.