This is fantastical history. It is loosely based upon real history, with a future loosely extrapolated from reasonably plausible events. But once you accept the possibility of time traveling teenage superheroes, you find yourself accepting a lot of other things (immortal vampires, costumed crime-fighters, magic spells, psychic powers, the list goes on) that have no basis in established fact.
Be careful then in interpreting what follows as any serious take on metaphysics. This is how it works in the game.
In general, what happened in the past was always going to happen. Time has many more dimensions than the lone forward vector that we can perceive. Those dimensions drive events like deep currents, and our little actions on the surface have little effect as to where the river actually flows.
To use the classic example, present a character with the chance to kill his own grandfather before that character is even born. In almost every case, he won’t do it. Maybe some circumstance prevented him. The gun jammed. His grandfather was wounded but lived. The character thought better of it, deciding not to chance wiping himself out of existence.
Or perhaps the thought of rolling a d6 for each and every item of the character sheet stopped him. In any case, killing your own father takes a rare and irrational sort of nerve.
What if he had the chance to kill Stalin in the 1930’s? How many lives would that save? (A lot! Stalin holds the record.) But since Stalin didn’t die in the 1930’s, something stopped that character, right? (Probably the XP).
We choose not to dwell upon the Butterfly Effect the way we choose not to dwell upon the plausibility of the Artifacts of Chaos. You can move a mailbox three feet and history will steam on unaffected.
Changing history requires deliberate effort. That’s a rule. You can’t do it by accident. Something will stop you. Because something did. That’s how it happened.