I’ve written about it here: Carriage Tolerances - Unusable Over Distance >75mm From Center - #34 by brent113
And there’s a more detailed discussion in this thread: Misaligned Layers, Nozzle Nudges and Thermonuclear Meltdowns - #47 by brent113
Just to be clear, I’m happy to hear you’re getting good bed adhesion. This will not affect that, and will require a new calibration to be run.
This just affects how square corners are, that is how close to 90 degrees everything is. For most things this won’t matter. But if you printed something tall, it would lean slightly to the side when you set it on a flat table, a small amount.
It’s just a bit surprising to me that support walked you through this whole process and never commented that the resulting bed level compensation matrix was a millimeter off from side to side, which is a really easy fix.
In my opinion, when you look at the resulting matrix using M420V the numbers should all be fairly close to each other. The bed level graph like you posted above should be pretty level. The slope across should be fixed, as something is out of whack there.
And again, this effect is small, you may not want to even mess with it. I just did a quick check using your numbers and it you printed an object 300mm tall a straight vertical wall would overhang 1mm to the side. That may be acceptable for the types of objects you are printing.
However, if you were expecting to print something with a fairly tight tolerance that needs to mate with something else, 1mm out of whack would be unacceptable. Since this machine is repeatable to easily within 0.02mm, it seems worth it to me to fix.