Board Thread:General Discussion/@comment-28415991-20160626005856/@comment-27195499-20160721231422

Well that's because there's an element of randomness to undyne's attacks so you can't really learn them. and sans is still the harder fight it's just that you have to learn his attacks after repeated failers which means that if you have beaten sans before you have muscle memory of his attacks which would allow you to beat him far faster than before.