I don't think the strips are joined with a SCANT 1/4" after all, even though the instructions specify that. If you use a full 1/4" does that help?
Because I'm at work (shh) I used post-its and notecards to simulate fabric, a desk ruler to measure the pieces and scotch tape to "sew" them together (after trimming out the "scant" 1/4" seams). I'm getting the same results you are using the same instructions.
I don't think it's you. I'd try one with a healthy 1/4" seam and see if that helps.