It's the way the system is setup. Rounds 1 through 10 all have slot values assigned to them that count towards the teams signing bonus pool. Rounds 11 through 40 don't count towards anything unless the signing is for more than 100k. Teams will draft seniors in say, Round 6, with a pick that has a slot value of say $400,000 and then they will sign for $1000, allowing that slot money to be used elsewhere.
Then you draft high ceiling guys in the later rounds who fell in the draft due to college commitments, and hope they sign.