Part prints too small, but 3D calibration cube prints perfectly  

Part prints too small, but 3D calibration cube prints perfectly

I have a weird one. I have a part that is printing too small in X Y and Z. Not by much, in absolute terms 0.15-0.4mm, but the part itself is small and is to interact with a non-printed object so the dimensional inaccuracy makes it unusable. The weird part is that when I print a 20mm calibration cube, it's as close as you could expect, 20.00mm+/- 0.05mm. I have done an XYZ calibration (and the mandatory first layer calibration) and belt tensioning but the error remains. The measurements in the modeler (OnShape) and the slicer (Prusaslicer) agree with each other, which points blame at the printer. And I like my printer and hate blaming it for anything. Now, FreeCAD, not so much. But I digress. I have had my Mk3S+ for years and printed hundreds of things and never noticed this problem before. Does this make any sense at all to anybody?

Ultimately, I can scale *this particular part* in the modeler to account for the error and force it to fit, but I would like to understand what is going on first. Because if I can't trust that *some things* I print will not print accurately, this will drive me nuts.

I tried attaching the STL file, but I am not allowed, since I am likely a shifty type, not to be trusted. Or being my first post on a new account. One or the other.