Deb, I have a hard time making size adjustments on blocks, sometimes. This one also confused me.
I made the first block with 2-7 1/2 inch fabrics and of course I knew that it wouldn't come out 12 1/2 inches but planned on just adding the 'frame' afterward.
My second try, I used 2 - 10 inch pieces of fabric and when it was done it made a block just a smidge larger than 12 1/2 inches... boy was I surprised. Have no idea how that happened but I needed a 12 1/2inch block.
This pinwheels is truly fast and easy and now I am seriously considering making more in floral prints for a quilt.
warm quilt hugs, sue in CA