Board Thread:General Discussion/@comment-32237043-20160626014030/@comment-86484-20161213113039

Check the books in the library. "Because they are made of magic, monsters' bodies are attuned to their SOUL. If a monster doesn't want to fight, its defenses will weaken."

Also, note how Mettaton's attack changes between encounters, even in his box form. And how Undyne couldn't land a single hit on Asgore when she was a child, but he's really easy to hit when you fight him. And how Toriel's stats in her battle are pretty low in spite of supposedly being one of the strongest monsters in the Underground (she doesn't want to fight).