Maybe your fabric didn't feed evenly? I have a built in one and notice when it isn't engaged when I piece. Things don't always feed as evenly then. I've been told to use the pin in the seam method (Stick the pin straight in at the seams AT EVERY intersection on the side of the block you are working on before you even try to finish pinning the block. After you've aligned the seams pin at a 90 degree angle about 1/2 inch on either side of your seam. (I've seen one of my in-laws actually baste the intersections on a problematic block.) Then ease the rest of the side together.
I'm not perfect by any means, I rip a lot but I'm really picky. I've found that my block issues that were that far off were usually a sign that I'd cut slightly off (ruler slipped a little, held the cutter a little off angle, etc.). I would go back and remeasure just to be sure. A real good sign for me is if the sides of the block aren't perfectly even.
By the way, my husband always tells me no one will notice but you. Except the last time, when I realized the stripes were running in the wrong direction. I think someone else would notice that one too.