Ber in mind that teams tend to have a backup board. The man board is the prospects they are targeting but if they're blown out on their main board, they'll dip into their backup board. I don't know specifically if the Pats use that but I wouldn't mind betting that's how the Dennard pick happened.
As for how the Pats draw up their board, I have a suspicion that scouts are given a set criteria on what the Pats are looking for, whether that be size, arm length, athleticism, character and even position and it's that rather than pure talent that helps populate the draft board across the full range.