In my previous response, I hadn't realized the sett was so large. After going back and looking at the apron on this, I was reminded of my Aberdeen District 5yd casual, and the discussion we'd had about the way it was pleated (neither to the sett, nor to the stripe.) The Aberdeen District, in 16oz Locharron fabric, has a nearly 23" sett size. I posted photos of my kilt in this post.

In post #16 of that thread, Matt Newsome explained what we were seeing. I've since picked up a copy of the Bob Martin booklet he referred to, and sure enough, I concur with Matt's appraisal. Perhaps the same thing is going on with the kilt at the start of this thread.