And according to the Draft signing tracker I follow, 30% or an avg of 9 per round, are unsigned from Rounds 17 - 20 thru yesterday.
The last few rounds like that are for backup plans. High upside types who you take there and throw a million + dollars to if something happens to one of your bonus babies up top.
The other scenario that happens is you take older players as org. filler and they decide to retire instead of sign for $10,000 or whatever.
There are 40 players in rounds 17-20 unsigned. 33 are HS or JC players who have quite a bit of leverage remaining or were the backup plans I spoke of.
The Cardinals took Cameron Johnson in the 20th round with no intention of signing him, but wanted to have a backup plan to throw money at if something happened up top unexpectedly.
Of the seven non-HS types, one is an Air Force player who wants to graduate from there. One is Tanner Witt, who was another backup plan, throw money at late arm for Texas. Derek Salata was a grad student who retired. Christian Little was another throw money at arm. DJ Uiagelelei was a joke pick as he's a QB for Oregon State. Kalae Harrison chose to play his senior year after a low-ball offer. Isaac Ayon, I do not know the story there.
Top-15 rounds = You're signing almost assuredly
> 15-rounds = Most likely signing, unless you're HS, then probably not