Look again at that small piece. You've missed something.
Well, now that I look at it in minute detail, I see some very odd patterns. I'm re-posting the photo below for reference. At the very top of the small piece, the twill of the red line seems to reverse itself every couple of threads, sort of like a herringbone pattern. But that doesn't match anything in the larger piece of tartan. So is this piece not from the same cloth as the larger one?

Also, what is that just below the folded-back portion of the small piece? It almost looks like a reinforced "keyhole" style button-hole stitch. Possibly even two of them, side-by-side. But I can't tell if the fabric was cut for the button-hole. Is that what this is, or is it just a repair?