I think they want you to look over your players and choose the best ones, and it might be laziness to not have reduced from the original 18. I put a couple of players on the transfer list, just to see if they would sell, and one did, so I had 17 players, and now I have 3 draftees for 20.
Also the normal weekly schedule is 2 league games and a training game. I've been trying to keep my starters out of the training game, so it helps to have some scrubs. I think 18 might be too many, but 13 or 14 might be too few.