Board Thread:Questions and Answers/@comment-32272607-20170612172318/@comment-27345308-20170612173420

That's because you're overpowered at that point. I'm pretty sure at some parts of Undertale monsters can even hit you so you lose more than your current HP and in a split-second negative HP appears before the game over.

Datamined monster data comes from Undertale's data.win file, which contains literally all the data in Undertale. If they were incorrect, that would be corrected by /r/Underminers long ago.