That's the first thing my mom said, check your 1/4 inch seams. I measured them, they were 1/4 inch seams.
I followed the directions, measured them, 12 inch block.
The site said a block is 12 inches, that's what I ended up with.
I just may have to put 1/2 inch borders around my blocks if I can't get them to 12.5 inches. I don't know what else to do.