Today I *FINALLY* found out the cause of a major Inkscape annoyance: 1px on screen never was 1px in document.
Turns out that correcting your "zoom factor" to match mm on screen to real millimeters #Inkscape thinks I certainly want to apply that factor when both; the document and display units are set to PIXEL. 🤦♂️
This drove me nuts for years, and I firmly believe the UI is at least partially to blame: