If I withhold my vote aren't I just reducing the number of possible votes to both candidates. In your scenario of 20 people voting, if one withholds his vote, then there are only 19 potential votes available to both candidates.
But one of those two people now has one less vote than the other, right? Do you understand this? Your vote is not a theoretical vote, it was for Candidate A or Candidate B. So if you choose to absent that vote, whichever it was, you have granted that advantage to the other candidate.
there are a lot of assumptions underlying US presidential elections because of how screwy our system is. If POTUS was elected by popular vote, we wouldn't be having this discussion, since Trump would've lost handily both times.
If, if you vote, you would definitely vote for Biden, then by withholding your vote, you are allowing Trump to win with 50% of the overall vote where Biden would need 52%.
Of course, that wouldn't hold if you haven't decided who you'd vote for yet....
Imagine votes as weights on scales. When everyone's vote is on the scales, representing who they prefer, it's roughly in balance. Then you remove yours.