I'm puzzled when I read that Vikings (7-1) is 2.5 winning games ahead of Packers (4-3). Shouldn't it be 3 games ahead? Where is the 1/2 coming from?

It goes in even games.

Lets use for example the Colts:

The Colts last week were 2 1/2 games ahead in their division. The Colts were 7-0
The Texans were right behind them with a record of 5-3.

So The colts were 2 games ahead of them but not a full possible 3. With the one game left over giving them a half.