Four jolly men

Four jolly men sat down to play,
And played all night till break of day,
They played for cash and not for fun,
With a separate score for every one,
When it came time to square accounts,
T hey all had made quite fair amounts,
Now, not one has lost and all have gained,

Tell me now, this can you explain?

The four jolly men are members of an orchestra hired to play at a dance.