Actually, that's incorrect electoral mathematics.
A switch from GOP to a third party is a one vote swing.
A switch from GOP to Dem is a two vote swing.
Consider this example:
A small town has 22 voters (any number will do). There are 11 Republicans, 10 Democrats, and 1 Independent voting for Mayor, along party lines.
If one GOP vote switches to Independent, the tally is 10 R, 10 D, and 2 I. That's a one vote swing, leaving the Pubbie and the Dem in a tie.
If a GOP vote switches to Dem, the tally is 10 R, 11 D, and 1 I. That's a two vote swing, and the Dem wins.