Just that 
I’m printing PLA on an Ender 3 v2 and slicing with Cura, and found the predicted filament length usage to be more accurate than the predicted weight of printed part. Is this just random or actually known?
Probably a close guess. The slicer has the length of the g-code path, so it can do the math and come up with the filament used (approximately) and can extrapolate the weight from that. Slicer estimates should be considered as “close enough is good enough”.
If you want a more accurate measurement of the weight you can also measure the density of your material and input that into the slicer under the material tab. This will make your answers more accurate.
The easiest way to do this would be to cut some filament of the spool into smaller pieces and weigh them out. Then take a graduated cylinder and add 100ml of water or so, dump your chopped up filament into the graduated cylinder and then measure how much volume they take up (whatever the current level of the water is at -100ml), then the density formula is density = Mass/Volume.
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