Board Thread:General Discussion/@comment-39394247-20190905145407/@comment-26907577-20190905150845

Well, stats are provided by the monsters.

GLYDE DEF HIGH ATK HIGH Refuses to give more information about its statistics.

It could be a matter of politeness to give them, but some of them are misrepresented for a few reasons:
 * Undyne the Undying doesn't know her own power
 * Asgore and Toriel are reducing their DEF because they don't want to kill you but give the real values.
 * Some monsters don't really know their stats.

Or maybe it's just a programming quirk to deliberately cause a certain amount of damage (and especially one-shots in Genocide).