I think it may be a combination of several of the things mentioned. After teaching beginning quilting for a couple of years, I will say that this is one of those patterns where any little error will be compounded as you go. There are no seams to match where you can ease things in, so they stand out.
One other thing you might watch. When you sew long strips, 'hold onto the end of the strip when it is sewn'. Sometimes it is easy to just let go and the end of the seam will not be 1/4".
