A Riddle

From "Perplexing Lateral Thinking Puzzles" by Paul Sloane & Des MacHale.

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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 a fair amount.
Can you this paradox explain?
If no one lost, how could all gain?

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