It seems that the 3D view within which I am working shows me the scene in a slightly angled / rotated way.
For example this TV Set is built from a cube, I never rotated it (just added cuts, etc). I added straight lines to explain what I mean:
It doesn't matter what object in the scene I look at, or how much I zoom back and forth or move things left or right, everything always seem to be slightly angled like that, even when starting scenes from a new file.
Quite annoying especially since I often align the camera to view, which then produces non-parallel camera angles.
Here is a GIF file reproducing the problem, right from the Startup File's cube: