Most UI fonts have tabular numbers. It’s number figures that all have the same width. Normally they are good in tables, where you need to compare multiple rows of numbers one under another.
But it should be used for time indication too! E.g. macOS kind of annoys me when every other minute all the icons suddenly move a few px to the left/right.
UPD: default clock on macOS 10.15 seems to not have this problem. I was using non-default clock.
Thanks @theLastOfCats for the left picture