Do not conflate functions of bed leveling with the bed mesh, they are two distinct and unrelated concepts.

The bed is level when its Z plane is orthogonal to the X and Y planes. The bed screws control and define the position of the plane. You will recall from basic geometry that three or more points form a plane, and moving the bed screws align each of the points of the bed to be of equal distance from the print head.

The bed mesh measures how flat the bed is as a map of its hills and valleys above and below that level plane. The plate is expected to be warped and not flat within a reasonable amount (normally 1.5x layer height is generally fine). When you adjust the bed screws it can create tension on the plate causing buckling and warpage and the flatness of the bed to change. A common cause of a grossly warped bed is that the bed screws are overly tight. However you can't just loosen or tighten the bed screws as that also raises or lowers the bed at that location.

You generally level the bed using a method such as the paper method (highly subjective, grossly inaccurate, yet works for smaller beds), or a semi-automated method like SCREWS_TILT_CALCULATE (accurate and preferred, necessary for larger beds). See https://www.klipper3d.org/Bed_Level.html

If the bed is overly warped (as seen in the fluidd Tuning Page and measured by the variance) then loosen the bedscrews completely and replace them with just 1 1/3 full rotations, then re-run SCREWS_TILT_CALCULATE to level the bed and re-evaluate.

…more to come, work in progress…