I have often wondered the same thing myself, and after close review of the images of them that I can find have discerned a few things:

The tartan pattern exhibited on the front apron CANNOT be used in any way to make the Saltire pattern on the pleats, therefore it must be some other special pattern weave for the pleated portions that makes the saltire possible. This conclusion comes from a couple observations, first that the saltire pattern is pleated to what appears to be a medium grey stripe over the fell and down the pleats, a stripe that does not occur on the front apron anywhere. Second, if you look at each individual pleat's contribution to the saltire pattern, most have ANGLED top and bottom margins to the white saltire segments ( one even has a pair of chevrons with one inverted at the actually crossing centrally), with several at either side of the pattern having flat tops but no further white extending above them in the same stripe. This makes no sense from looking at it as a standard tartan pattern, as these configurations are NOT possible with symmetric or even asymmetric woven tartans. So the material making up the pleated sections has to be of a different pattern than the aprons.

This should also apply to the other variations of this "pleated pattern" kilt such as the rampant lion in grey and red, and I believe there is also an english specific color and pattern scheme as well.

Okay, does my logic make sense to anybody but me? I personally would love to see one of these kilts deconstructed to see the actual pattern fabric used for the pleats, and assume that I would see a fabric joint on each side where the standard tartan aprons were joined, probably in a deep or reverse pleat.

jeff

Edit: Obviously Steve and I were writing our posts simultaneously and he pushed his send button before me.