Looking at the picture again, more closely, Richard, I see that it is not just these two stripes in the rear of the kilt. If you look at either side of the kilt, just beneath where the buckles are, you will also see the same stripe configuration. So it appears that this is a regular, repeating pattern that the kiltmaker used.

It could be that the kiltmaker was simply trying to be creative. Maybe the original client said, "You know, I don't want this pleated to the sett or to the stripe. I want something different -- be creative." Who knows?