It's hip to be square

I have glued up two pieces of the chair. Today I un-clamped them, and put a carpenters square against the joints....

Ooooooo baby, dead square!

Both are perfect 90 degrees.

This a real anomaly. Pretty much with every project I make things never come out perfectly square. Sometimes they're off by just a hair (or more). I think the glue I use has too many glue gremlins in it, that start moving things around once I tighten the clamps. Yeah, that must be it. Problem solved.