Board Thread:Questions and Answers/@comment-39463658-20190514115617/@comment-1327106-20190514213445

Keith sounds pretty certain in the anime, but there is likely some rounding involved, otherwise Grisha lived too long. Given the the possibility of Keith rounding or Grisha experiencing a miracle, it's more likely that Keith simply rounded the year up.

Listening to the Japanese audio in the anime, Keith is very clear on the twenty years ago date, but in the manga the translation reads, "I met Grisha... twenty years ago, now?" which implies a lot less certainty. Manga Keith is likely taking a rough guess at how long it's been, and rounding eighteen to twenty is not that far off, especially given the span of time involved.

It's likely the screenwriter for the anime didn't realize that removing the uncertainty creates a problem, or the actor didn't realize it and thought that making a definitive statement made for a better performance.