They probably have you cut that way to avoid seams. One thing to remember about mitered borders (in case you never did one) measure the quilt across the center and use that exact measurement to sew onto the side (1/4" in from the edges) but don't forget to leave the overlap on EACH end so you can miter the corners. I did that wrong twice. Once I didn't have enough overlap to miter. The other time, I didn't measure the center and my border got wavy.