Suppose the visitors trail 18-14 and score a four-point try. They draw level at 18-18. A successful two-point conversion would make it 20-18. The kick changes the margin without adding another try.
A 20-18 result is a two-point win. It covers -1.5 but falls short of -2.5.
A points total combines tries and scoring kicks. To count tries, use the scoring plays themselves, especially when the score includes penalty goals.
More sports: Rugby Union, Australian Football.