...pleated to the stripe. The reason...it's easier.
To be honest, I think pleating to the stripe is actually harder. Yes, it's easier to lay out the kilt and figure out where to put the pleats, but it is harder to make the pleats look good. The main stripe has to be absolutely centered from bottom to top, and each pleat has to be perfectly tapered from bottom to top. Unless you've made a lot of kilts or have a _lot_ of patience, it's hard to do this perfectly. I still prefer pleating the the sett and can make a kilt faster if it's pleated to the sett, and I've made zillions of kilts.

On the question of why the tartan is not square: it ought to be because tartan is what weavers call a "45" weave - the twill line should be at exactly a 45 degree angle. Some handweavers adjust the threat count to make squares absolutely square when the tension is off the warp. In commercially-woven tartan, squares are never square, in my experience, and the warp-parallel dimension of the sett is longer than the weft-parallel dimension. So, if you're figuring out the sett, measure parallel to the warp (the long dimension of the tartan).

And a sett is the full repeat of the tartan, where you can start over again and get exactly the same pattern of stripes. A tartan can repeat A-B-A-B (like the Wallace or Rob Roy) or A-B-A-C-A-B-A-C like the Black Watch or even A-B-C-D-A-B-C-D (an asymmetric tartan like the Ancient Campbell of Argyll). The full sett for an A-B-A-B tartan is A-B; for the Black Watch, it would be A-B-A-C. You could pleat to the stripe every half sett if you pleated to the "A" block or to every full sett if you pleated to the B or C blocks. That's what chasem is talking about with the Forbes.

Barb