Bed or Gantry bowing

Any idea what might cause the bowing? I've tried loosening and reattaching x-gantry as well as y-rails since there seems to be bowing in both x and y directions.

The curving seems symmetrical and adjusting corners only increases variance.

I thought that maybe the spring steel print surface might be curved, but turning it around gives the same results.

I have a bed plate from Ratrig