You have 2 blocks one is 10 1/8 square and one is 10 4/8 (10 1/2) square. So you have 3/8ths difference. So you need sashing strips that are two different sizes one is 3/8ths narrower then the other. You add the bigger sashing strip to the two sides of the smaller square, and the smaller sashing strips to the bigger square. Now your over all squares are the same size.
10 1/8 + 1 3/8 = 11 4/8 and 10 4/8 + 1 = 11 4/8.
Since you are only sashing the two sides (right and bottom for instance) those sashings never meet so to the eye w/ just that little difference it won't be obvious.