The pattern does state in the skills required: "All straight seams and straight edges; decent 1/4" seam accuracy with attention to not stretching strips." Unless the jelly roll was cut incorrectly, I'm guessing your seams may not have been consistent. I would take a look at the seam on the narrower side of the top and see if they are wider than the other side. If so, you may be able to unsew and re sew them. You can also try taking off several rounds and see if you can square the center then re attach them. As you mention, you really can't trim this into square.
You are not alone with this. Even normal sized log cabin blocks can get wonky. You need to square those as you go and it helps to measure and cut each log as you attach rather than sewing the strip then cutting the excess.
Last edited by PaperPrincess; 11-02-2015 at 03:08 PM.