There are two answers to that; both the same. One them is "simple" fact, the other one is the same with a whole lot of explanation and maths and all sorts that, to be honest, I wouldn't trust myself to get right even given a day to dig it all up and write it out, and ultimately it finishes in the same place anyway. So I'll just give you the "simple" fact version, without supporting documentation, I'm afraid.

Here it is; because that's how the universe works.

That decision doesn't get made until "something" checks it/depends on it/interacts with it (the English language, at heart a way of telling macroscopic monkeys about tigers and bananas, does struggle a bit here). If you go digging yourself, you'll be able to go as deep as you like. I apologise for not presenting a better answer, but I'd really be doing you a disservice.

Edit: It's really not a good answer at all. I'm digging through old reddit threads to see if RobotRollCall ever talked about this. Her explanations were generally very accessible.