As a textile proffesor I would like to mention that the reason you can make this change is that you are moving the grainline. Yarns in a woven fabric hang vertical and will try to fall toward the ground (following gravity like a plumb line). When there is a flair the fabric will try to fall along the vertical grainline and create a ripple in the excess. By moving the position of the grainline you can change where the fabric will fold.

This article shows a simple method of moving the fabric around the grainline to create the effect you want. To test this theory: Cut one skirt front from with the grainline down the center front, cut one with the center front on the bias and a third with the straight of grain at the side seam. By manipulating the grainline you will see how to make a flaired skirt appear slimmer or wider by the placement of the ripples.