There are several ideas about collections:
1. [ 100%] Collection, which automatically adds games in which you have received 100% of achievements;
2. [ ex. 100%] Collection, which automatically moves games from the [100%] collection, in which you have received 100% of achievements, but an update or DLC was released, and there are more achievements and you no longer have 100%;
3. [ achievements added] Games are added that did not have achievements at release, but achievements were added over time (a week / month / year later).