First of all, a player must match a system. For instance: is Chubb a terrific (or has the potential) pass catcher out of the backfield? Because if he isn't, there is no point in taking him as McVay's system needs a RB with excellent receiving skills.
That goes for every player and every position.
Further, typically once you get past the first 10 or so players, players start to cluster. Several players will have similar grades. Similar does not mean identical. But close enough that one doesn't have a clear higher value over the others in his cluster. So, you take the player from the cluster that best fits your system AND fills a need.
Might a player that was predicted to be taken higher fall? Of course. But even then, you don't take a player that doesn't fit into your system unless that player is so extraordinary that you'd be willing to redefine your system to accommodate him.
That's not going to happen around pick 87.
This is why BPA makes a better bumper sticker than it does a draft philosophy.
