You had to invest 10 credits to start the game, so when you win 40 credits you gain 30 credits.
Too the pot is shared: the winner gets 70% of the pot. The winner of a 10 credits game gets 28 credits.
And sometimes the pot is even more shared: when two people have the same points, they share the credits of 1st and 2nd rank:
28 credits (1st place) + 8 credits (2nd place) = 36 credits shared by two player = 18 credits each

The same can happen with 2nd or 3rd place, and 3 or even 4 player can share the same place.