See the post yesterday for details - I just got an email from the mystery designer who realized that she had written the instructions to allow for trimming and then didn't tell the user to trim to size. That was the problem with the dimension discrepancies.
What a relief - I'm not nuts. Things like this really test your sanity when someone verifies they are looking at the exact same inputs that you are using and then comes to completely different conclusion.
On another front - this is what I get for being smug. I'm churning along on part two of Bonnie Hunter's Quiltville magazine mystery, thinking to myself how quickly I'm completing this step when I remember that I was planning to make 25% more of all the pieces to increase the quilt to king size instead of twin size like the instruction. So I'm not almost finished after all. Back to the cutting table.