When fabric is wound on a bolt, each wrap as you move outward is a little larger than the one preceeding. That also holds true for the outer portion of fabric that is folded in half, as most fabric is. The effect adds up, so by the time you have ten or twenty or more wraps on the bolt the fabric becomes misshapen, and it becomes more permanent, and it doesn't necessarily want to snap back into shape once it's cut from the bolt. That's why you get a diagonal frayed edge sometimes on a piece that you've bought. It looked straight when cut, but it didn't follow the weave. That's also why some old timers like me prefer to have our fabric torn rather than cut, so you actually get the yardage you think you have bought once you take it home and align it properly. Prewashing and drying can go a long way towards getting the fabric to relax back into its original woven or knitted shape.