So the game assumes that what your character becomes is what you were training for. It says this in the multicasting section so that there isn't confusion on how a fighter all of a sudden can cast spells (because the player decided to take a level in wizard). I think the reason they put most subclasses at level 3 instead of 1 is for new players. It's less confusing if a new player says I want to play a fighter, and not have to worry about what kind of fighter they want to be. Once they get the hang of playing the basics, then it becomes, what kind of fighter do you want to be?Oh I totally agree that everyone should choose their subclass at level 1.
I guess you could hand wave it as you were practicing these spells and magical effect on your spare time all along and they just NOW start to work.