The weight is traditionally specified as ounces per linear yard. So, eight times 13 is roughly what you will get if you have 8 yards of tartan. However, the 8 yard kilt you have is really only FOUR yards because it has been split down the middle. So the new maths is 8 x 13 / 2 = 3.25 lb which is only 6 ounces different to what you weighed

Why the difference? Well, the length of the kilt may be less than full half width to start with and that will lose a good few ounces. Then the buckles will add some. Finally, your weighing equipment is unlikely to be absolutely correct. But not far out though and no surprise as long as this is wool