I wrote a gcode file to move a digital dial indicator around my printer's bed to check the level and flatness. I start by zeroing the gauge at the center of the bed. All values displayed are relative to that starting point. Notice the bed has a slight hump in the middle, or taking the other point of view, the corners sag relative to the center and the right side, which has no middle support like the left side does, sags most. This test conducted with the bed at about 15C.
The bed is 1/4" cast aluminum tooling plate with a layer of 5 mil kapton tape. The measurements shown obviously include variations in the thickness of the tape, its adhesive, and anything under it such as trapped air bubbles, dirt, etc.