Not going to pretend i know all, im in my 2nd and 3rd season respectively, but it seems like playoffs are ripe for upset because niche players become more useful. Those "stud" pitchers with poor durabilty suddely become very useful. Another factor could also be injuries. Maybe your opponent dealt with key injuries during the season that kept that record low, but healthy at the right time. Along the same lines would be mid season trades or calling up a stud rookie, wouldnt necesarily be reflected in win/loss record but would have an impact in a short serise. Yet another factor could be fatigue, either from ur player or them gettin 100% rested.
Just some possible factors, could be that none of these apply, but worth considering imo.