Those are fairly small squares for a 10-1/2" block. The discrepancy could be simply in how the seams are pressed. If the pressing of 8 seams takes up just 1/16" of an inch extra each, that will add up to a 1/2" discrepancy. Look at the back of the block and examine the seams to see if there is a difference between the seams in the short direction and the seams in the long direction.
Re-pressing the seams may help. You can also try to "block" the block to correct dimensions. Sharon Schamber has a couple of Youtube videos demonstrating blocking techniques. Here is the one I think you would want:
http://www.youtube.com/watch?v=sQkF02vpVuw