After bitching about the chiwanese chairs I was repairing not being square, I confess to a senior's moment (even though I'm not Mexican). The back of the chair is 2" narrower than the front, by design.
A stupid mistake on my part. Have to remember to measure all 4 sides next time, not just the front and one side.
My son/shop assistant got a good chuckle in at dad's expense when I realized what I had done wrong...