DOH! Here's the lesson I learned today. I put together the first two of my blocks and was extremely disappointed to find that they were both 12 x 12 instead of 12.5 x 12.5. I have never had this problem before, and was quick to blame the pattern.
After going through the calculations, everything should have been fine with the pattern, so I started investigating other causes.
Bottom line - when my machine recently came back after routine cleaning and maintenance, the tech put on the #1 foot instead of the 1/4" foot. It never dawned on me to check.
There can't be more than 1/32" difference between them, but in a block with a lot of seams, it made a significant difference.
Now, back to work.