For the most part, the NFL Combine is for checking boxes. In other words, it’s about confirming what we already know about a draft prospect from their tape. After all, if someone’s physical attributes don’t transfer to the playing field, they aren’t very relevant to the conversation of what makes this person a good football player.
In these cases, it doesn’t make a lot of sense for a player to drastically move up and down your draft board after the NFL Combine. They either confirm what you already know about them, or alter your expectations a bit and make you go back to the tape to see if you missed something.