It used to be (and may still be) that if a post was deleted by its writer, then all the other ones subsequent to that post moved up to fill the gap.
So if post #50 in a 100-post thread is deleted, posts 51-100 become posts 50-99.
References to previous post numbers that were typed in manually could then be out by a few if one or more intervening posts were to be subsequently deleted.
The permalink feature (button next to the post number) is I think supposed to avoid this, by ensuring that deletions and resulting changes in post numbers are tracked automatically.