Depends if the story is an isekai (portal fantasy, for those who don't speak weeaboo) ...
If the story is an "isekai" and only the MC sees the numbers, and MC is supposed to have powers in the first place, you could blame the entire thing on his modern brain translating magic in the terms he understands ...
Because, honestly, how do you explain to the brain that there is supernatural force bending reality?
Well, other than schizophrenia, or use of hallucinogens, of course.
If the protagonist isn't supposed to be insane or intoxicated, isn't from our world and magic is real in-universe, and everyone sees the number, the popular explanation in fantasy is the "game of the gods". Gods made it this way. Why? Understanding the reasoning of the eldritch entities from beyond the reality is usually quite difficult, so no one knows.