Riddle #14: Four Men

This one's a little different, but kind of cute. (No, I didn't write the poem; it's from a book.)

Four men sat down to play.
They played all night till break of day.
They played for gold and not for fun,
With separate scores for everyone.
When they came to square accounts.
They all made quite fair amounts.
Can you this paradox explain?
If no one lost, how could all gain?

NOTE: The correct answer has been guessed. See the comments section for the answer.

Lateral Thinking Puzzlers by Paul Sloane has lots of classic riddles, clues and answers. Click here to order it.