1/2 games occur when teams haven't played the same number of games and one team is on an odd number.
Let's take the two teams in a standings:
Team A 7-2
Team B 5-5
Team B has two less wins and three more losses which equals five. Now divide by two. The result is 2 1/2. It would take three wins by Team B over Team A to take a 1/2 game lead on them.