It must be a mistake in the measurement of the C & D block. Since the C block is supposed to end up 6, the D block ends up 7, and the sashing is 1.5 inches, equaling 14.5 inches, but that doesn't allow for seams. The finished B block is 14.5 inches. If C is supposed to be made with 1 3/4 squares instead of 2 3/4 inch squares, then that should be 6 inches, but there is still a problem with the C/D column.