Easy. After you've seen how much more you need to win, you can now check the registration lists, see who hasn't voted, and assign all the fake ballots to those voters. Unfortunately, that takes a little bit of time to complete, and you can't have pesky poll watchers present while you do it.

EDIT: Graph taken from this insightful article: https://www.sgtreport.com/2020/11/the-definitive-case-proving-donald-trump-won-the-election/