Most of the other answers have focused on the right answer to talk to the other players like adults. All good. But what happens in game?

Make the mistake part of the story you are telling. Find an explanation why said power was (is) more powerful for that one character. Is it a blessing from the gods? A curse from some demon? Something even more nefarious?

  This way, you get some extra story without having to retro fit lots of adventures.

Most of the other answers have focused on the right answer to talk to the other players like adults. All good. But what happens in game?

Going back and retrofitting one session is hard, many is nearly impossible. You could start the game again but I have never seen it work. Playing English games when our understanding of English was basic, we had to retro fit many such errors. What we did was a mixture of retrofit, use the correct rules from now on, and making the mistake part of the story. The latter always worked best as it gave us more story to work with.

Therefore, I suggest you could make the mistake part of the story you are telling. Find an explanation why said power was (is) more powerful for that one character. Is it a blessing from the gods? A curse from some demon? Something even more nefarious? This way, you get some extra story without having to retro fit lots of adventures.