And now the mystery thickens ... prompted by your last post, Tobus, I looked carefully at the difference in the fabric on the turned-under single hem and the front of the apron ...

Here's what I noticed ... on the portion of the fabric that would be the bottom of the kilt if the hem were removed ... the black "stripe" in the set that goes horizontally across the sett is about 1/2" wide ....
IMG_3260.jpg

But on the remainder of the kilt as shown below ... this same horizontal black stripe in other corresponding places on the sett is almost 1" wide.
IMG_3261.jpg

So ... it does appear that some alteration of the sett has been done at the bottom edge of the kilt, which is now the turned-under hem. Does that confirm your instinct that the sett might have been altered on the loom to create the selvedge ...?