Square at opposing corners, but

Hi guys, I was in batch 1 and just now getting to build the table. I just got done mounting the water pan, and decided to check my diagonal frame measurements. Two opposing corners are square, yet the diagonal measurement is about 3/8 off! Both y-axis carriages touch the plates at the same time, all the gantry fasteners fell right into place, and the base frame tubes from end to end are equidistant from one another…
How can I have opposing corners square, yet not BE square?

The most likely culprit is probably warped tubes.