Or they even say this is a sign of wear and means you should throw it out.
And because I enjoy sharing knowledge more than boasting I know more than others: the reason it moves is to account for the thickness of the metal hook itself.
It makes a difference if you are hooking it onto the back of something and measuring from there, OR butting it up onto something and measuring from there.
If you want accurate and consistent readings in both of these situations, the hook has to move. It basically pivots around the true point you’re measuring from.