It is a non-repeatable event, though, or a highly improbable one, at least. We don't know the exact way in which the universe collapsed on her finger, the exact distribution of one part or another. Maybe it's the way she's taken through; maybe it's the timing with which she was parted. We can't say that simply lopping off your finger this way does the trick. There's probably more to the -way- it was lopped off than that. Again, this is phenomena no one in the game understands. All we now is that, in some way, it was linked to the moment where she was split between timelines.
Also, the explanations for Marvel superheroes are terrible. Might want to pick a better example and not point to fiction that actually is terrible.