That's because you placed the 3 bets at around the same time, and at that time you had about 5,000 DOGE in your bankroll, and since the max bet for the competition is 5%, only about 250 DOGE were counted for each bet.
The mistake in your calculation above is that you assumed 500 DOGE bet, but in fact only 250 were counted.
The 5% max bet size is determined at the time of bet placement and not at the time of bet settlement.
Had you placed each bet after the previous one got settled, then your calculation would have been correct.
Does this clear it up ?