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...