Why should the fence be mounted on the glider so that the "A" end will pass through the photogate first?

Question

Answer

Because individual picket spacings may not be exactly equal, this allows one to read the position of the glider accurately and directly from the fence calibrations in Appendix C.

Because only the edge labeled "A" will trigger the photogate.

No special reason. As long as all measurements use the same leading edge consistently, it doesn't matter which edge it is.