Here’s one way to think about it: There are two sources of uncertainty when we model predictions for political elections and sports: incomplete information and true randomness.
First, there is information we do not know or cannot model. In politics we do not have the ability to sample all voters in our polls or eavesdrop on every candidate’s strategy meetings; we can only speculate. In sports, likewise, we do not know the true state of a player’s health or the coach’s strategy for the team.
Second, there is randomness as to what will happen during the course of the event. In politics this includes private remarks that are captured on camera or whether a candidate knocks a question out of the park or flubs an easy one during a debate. Even the best politicians have bad nights (think of Gore’s disastrous 2000 debate in Boston), just as the best receivers drop an easy pass every now and again (I am looking your way, Wes Welker).