My guess would be that the number reported was the biggest (supposed) turnout of Democrats in Wisconsin history - and that sort of enthusiasm for Biden should have been easily anticipated based on the incredible turnouts at his campaign appearances.
(that’s sarcasm for you folks still exhausted by recent events).
You’d think then you would have seen the exact same thing in Ohio. Didn’t happen, did it? Trump won it easily, and the count was finished quickly.
The difference? Republican governor.