It's because you're going to end up making half square triangles that will end up being 3 1/2". Since you always have to trim them up anyway, it's much easier just to cut 4" squares, mark a diagonal line on the back, put the two you need faces together and sew 1/4" on both sides of your line. Then just cut on your line, press them open and trim to 3 1/2" square.