Well when you lock in a subclass it creates a fresh version of that subclass this fills the vacant space. For example, Jimmy has a Voidwalker subclass made for PVE. He levels it up completely and wants no further changes. He chooses to lock the subclass, gaining a boost to recovery but prevents more changes. However, when Jimmy looks in his inventory he finds a new Voidwalker subclass. Jimmy puts his old subclass in the vault because it is not needed. Jimmy has to level it again drone the start, but this subclass levels slightly faster. Oh no! Jimmy made a mistake but locked his subclass. It is good that Jimmy put his old subclass in the vault so he has room for the new one. Now Jimmy has three Voidwalker subclasses. Jimmy learned that the extra space is for the new subclass not some crazy arc one.