I think even 150 years ago, there was "early voting" in the US.
For example, if the election was on November 1st, with the slow pace of news transfer, etc., people's last interaction with a candidate may have been from September.
People are voting based on that particular point in time. Probably evens out.
People vote for Candidate A a month early. In the next month, sometimes Candidate A looks much better, sometimes much worse. People who voted for him wish they hadn't. People who didn't vote for him wish they had.