I get what you're saying, but I don't think it's a cut and dry situation. Current needs are as much about where the lowest ratings are, and where the current players are in their contracts as it is about simply missing a player, and what players are available that match their scheme and their needs.
Just because it says they need a SS the most doesn't mean they'll use their highest draft pick on a SS, if there is an obviously much better player on the board at that time that can significantly improve their roster - especially if it's a STAR or QUICK development. It may be that there aren't any great SS on the board that match their scheme commensurate of the draft round they are on.
So just because the HB on the roster isn't one of the three lowest rated players on their roster, the ability to draft an "adrian Peterson" quality back that is scouted as such, would not get passed over if doing so will still improve that position, with the notion of where it can advance to, and replace current expiring contracts, etc.
IF a teams biggest need is SS, and all things equal their draft pick comes up and there's a SS and four other players all available, all similarly rated by scouts, and all match their scheme, they'll then default to their needs.
Besides it's easier to improve and replace certain positions in later rounds than others.
That's what I've seen anyways - usually. Sometimes the logic is haphazard and seems random and gives a WTF moment. But typically I see them select who is valued the most on the draft board at that time, that will also improve their current roster or replace an expiring contract if their needs are not able to be met at that moment - or there's someone who will and fits their scheme better in the next round.
Robert