They do a base calibration. Do a Google search for Dell Calibration report. It shows what they calibrate. Keep in mind, it's still a pretty loose calibration. I've never once gotten two of the same models that look the same out of the box.
This has driven me nuts more than once. I’ve very intentionally ordered two “matching” monitors at the same time, supposedly from the same lot, and they were vastly different. It may be an overreaction but since a number of years ago I’ve gone to just one big and nice monitor (even if it’s not technically as nice as the others) because those details get in my head so much.
As the saying goes: “comparison is the thief of joy” - so I remove the second thing to directly compare to.
I am under the impression they do some basic tests that don't entirely accommodate for uniformity of illumination and consistency of color across the display. In other words - I think maybe they only test one part of the display.