The short answer is: math.

The long answer: If you, for example, win a game with four other players in the 5 Credits room, you get the 15 Credits stakes from the other players. But you invested 5 Credits to enter the game. So while you are playing, 5 Credits are deducted from your account. When you win, the 20 Credits are added to your account, which increases the Credits you own by 15 in total.