When you cut carefully into the selvedge of a fabric, and then yank the two edges apart, the fabric will tear straight across the grain of the fabric, breaking between two horizontal threads.
The problem is that fabrics are not always wrapped perfectly straight onto the bolt. If you have ever bought a plaid fabric, you may have noticed that the clerk cuts a single layer at a time, on the lines of the plaid, rather than cutting both layers at the same time. That's an illustration of this same issue. If you cut through both layers of the plaid fabric at the same time, it will look great on the top layer but the unseen bottom layer will probably be crooked.
Did this help?