You lost 30 credits (10% of your credit amount) to WGT as part of your entry fee.
When you won, you won your opponents credits (270).
You also won back your credits, but you didn't get back the full 300. You only got back 270 of your original 300. You lost 10% to WGT, same as your opponent.
If you started with 1000 credits.
1000 - 300 = 700 Cost you 300 credits to enter the match.
700 + 270 + 270 = 1240 You win the match. You get your 270 in winnings and 270 of your original 300.
It looks like you only won 240, but you did win the full amount.