So the reason for the collection being completed without 380 cards I believe has to do with players being added to rosters.
Using round numbers for the sake of ease, let's say you have collected 300 of 380 free agents. If 50 of those collected cards happen to end up on a teams roster, you'll only have 250 free agents left in that collection, but the game will credit you with collecting 300. Collecting an additional 80 would now show you at 320/380, but the game will credit you with the full 380 because of the previous 50 collected.