AS fluter and sydnie7 have told you, you are entirely correct. I compensate (usually) by making the hem a bit deeper inside the under-apron pleat.

You can also just blow it off, this is a quick, easy contemporary kilt, not a traditional. You've realized there's an issue at that point. You've learned something about kiltmaking, and are a little step closer to understanding what goes into making "real" kilt or a first-class contemporary garment......the authors goal is accomplished.