What kind of math are you talking about? Not voting for Romney is not a vote for BHO it is a non vote for Romney.
Think of it this way. For Obama to win, every vote for Romney has to be cancelled out by a vote for Obama. Then Obama needs just one more.
Now let's say that you and I dislike Romney so much that we refuse to vote for him (that's quite possible!).
What's the result? Two Obama votes aren't used up cancelling our two votes out.
Our not voting for Romney has given Obama two extra votes.
A non vote for Romney translates into one less vote Obama needs to win. That's simple logic which is a math topic and it seems to elude you and others.