re: Games above/below ".500"

This is a semantics problem. 20-10 team is 5 games ahead of any team that is sitting at 0.500 regardless if they are 1-1, 15-15, or 30-30.

But when you say over .500, .500 is not a team that will accumulate wins and losses. It is just a milestone that you are holding that singular team against. They are therefore 10 games ahead because they have accumulated a 10 game lead against that milestone.