This.
Draft results are more an indication of consensus popularity than they are any purely objective ranking of "talent", regardless what any fan thinks. And, any college prospect acquired during draft weekend immediately becomes an NFL rookie, so their draft status doesn't (or shouldn't) matter anymore except with regard to their cap hit (although it seems to matter a great deal for a few other teams - especially for the perennially bad ones).
Historically, prospects who are signed as UDFA have had, percentage-wise, better NFL "success" rates than 7th-rounders in terms of total games/snaps played in all three phases (plus statistical contributions). The prospects who were drafted in the 7th were simply more popular at the time among the 32 teams than the guys who went undrafted.
Of the 63 players who have played active roster snaps with the Pats this season and helped them get back to the Superbowl for the 2nd consecutive year ...
-- 20 were originally drafted in the first three rounds (regardless who drafted them)
-- 21 were originally drafted in the 4th thru 7th rounds
-- 21 were originally undrafted
*** This roughly 33%/33%/33% ratio is typical of every NFL roster, every season.
The current active Pats players who were originally UDFA include:
Ryan Allen
Amendola
Andrews
Bademosi
Adam Butler
Malcom Butler
Develin
James Harrison
Hogan
Hoyer
Jon Jones
King
Eric Lee
Waddle
*** EDIT:
Just to be clear, "typical" wasn't intended to mean absolutely every NFL roster, every year - just on average. For example, the Eagles roster breaks down as follows:
23 players drafted in the first three rounds
21 players drafted in rounds 4-7
10 UDFA
(total 54 players who are listed as having been active for games this season).