You only came 3rd, the winer only gets 405 credits, so 3rd is much less, check it out yourself, you must be mistaken with the amount you mentioned above.
The winner gets 18% of 2250 credits which is 405. You would get 6.8% of 2250 which is 153 credits + you wont get that much either because you were tied for 3rd place, so i don't know where you've got the 1800 from????