Oh, I'm so glad to know that I'm not alone. No matter how carefully I cut and sew, I'm always off somewhere and I can never figure where I got off. I remember my first teacher warned us that if you were off one-two threads on each cut, it could make you off by an inch or more and so I've tried to remember that and usually try to sew the scant 1/4" seam to compensate. But like you, it never turns out exactly right. Just remember: "Who can tell the difference on a galloping horse."