Library, Depot and Shores are trophies dependent. So if you got into Library means you are of certain standard and thus get matched with players.of the similar.trophy level. This discounts droppers, players who really only know how to overpower and just luck (good/bad).
The flaw lies with some players just buying boosts and then getting stuck in upper Library due to bad boost configuration or just being bad at the game. These players are within 200 to 400 trophies of legit FTP, low boosted players so they get easy matches then meet a proper boosted team and get kicked back.to Library.
The question now is why the range of matchmaking is so large and why such players are unable to escape away from the pack.