If you look closely at the picture, the square to the left of the quarter is slightly larger than the quarter's square...I think this so often comes down to the accuracy with seam allowance. I deal with it myself. You'd swear you sewed the seam accurately but the non-match proves otherwise. Maybe stop periodically and check your individual squares before you sew them into rows. I'll bet you that you find one or two that are a tiny bit off and they are your villains when it's time to put the rows together.