It's not that "multiple pasts can exist", it's that "multiple pasts _would be sufficient to explain_ the effect that is being observed now".

E. g. "The apple hit Newton on the head" could be explained by:

1. The apple was hanging on the tree at `t - 1` and the wind shook it loose.

2. The apple was flying through the air because his mother threw it and it collided with his head

3. The apple spontaneously generated due to as-yet-not-understood quantum fluctuations 3 meters above his head.