I have an explanation.
Most clocks and watches lose (or gain) time over, well, time. If one sets the exact time, a few weeks later the clock may be slow and the person may miss something important. I think the “ma’ariv phenomenon” is well known. You think you’re on time, but when you get to shul, they have already started and you missed borchu. Setting a clock or watch ahead solves that. Yes, there is a psychological element here too and it works for many people.
I find it difficult to compare this to the issur of owning inaccurate scales.