What seems to be the issue? If you got those mesh numbers using a probe, it just indicates that your bed is unlevel. You can home it, then disable the steppers to move the hotend around, and do the paper test. If you haven’t leveled your bed before then you can google the paper bed-leveling test.

I say this because it seems like your bed is slanted downwards on the bottom-left corner (and/or slanted upwards on the front right corner). No offense if you do know how to level your bed though, if it’s some other sort of issue do let me know.

Oh yeah I mean it’s definitely not level. The 3 pro doesn’t seem to have a way to manually level it. There are eccentric nuts which I have tightened, and that did stop the bed from rocking. But I think this is the “natural” lay of the bed. It seems like the disparity is vast

Wasn’t sure if there was something that I’m not seeing on how to fix this on this particular device.

Gonna preface with saying I don’t have a Neptune… so this is from checking out photos.

First thing to check is how square the printer frame is- especially how the gantry is to the rest of the printer.

If you can, take a big triangle square to do that. You might have to loosen and retighten things, in that case go slow and tweak them all one bit at a time. You might also not ever be able to get it perfect. (I assume there’s a t-brace going between the gantry uprights and base. Hopefully it’s not just but-end bolts…)

from there check squareness if the x-gantry itself. That may be out of level and need adjustment.

Doubt that would solve a 3mm differential though.

Next it looks like there’s stand off pegs? You can take the tall pegs off and sand them down a bit until they match the low side. Tedious and annoying, and very easy to go too far. Before that try tightening the high side.

Maybe check them with a micrometer but ultimately every pass or two you’ll have to put them back in- it’s the carriage+posts+plate/bed altogether that’s out of whack.

And this is why I dislike rigid legs like this. Yes springs go out of level routinely, but the pegs will eventually go out of level all the same and it’s a PITA. Or you change some configuration and have to do it again…

Regardless, you should be able to set fade height to compensate for it. With say 10mm of f-height, the firmware will automatically adjust layer height until it’s square and flat at 10mm. If it’s absolutely critical for dimensions or appearance, you can set a 10.4mm raft.

It has pegs but I was able to see that there are rather typical screw heads in the heating plate to hold it down onto the bed's support frame, and if you look you can see the bolt ends coming down out of the underside of the supporting arms.

So I think what's here is pretty simple: regular bolts with fixed length spacer tubes around them.

And that means that OP should be able to sort this out with, say, the right collection of 0.5mm-thick washers. One on the front left screw, three on the right rear screw, uh, six on the back left screw should get close enough for the mesh to handle the remaining difference.