I once made a quilt that alternated a start with the short block and then the next with the long block. Each row had the same number of short blocks and long blocks, which I had measured very carefully and cut out with a rotary cutter. When all was said and done, every other row was 6 inches short!!! I puzzled over it for the longest time, and then called my husband in to help me figure it out. He's a land surveyor, which means he measures land for a living. Even he couldn't make sense of it! We measured everything and never did figure out what the problem was. I ended up trimming the long rows and adding some sashing, and no one ever knew!