When wagering with a bonus, certain restrictions and limitations apply — the reason for which one of your bets might not have counted towards the fulfilment of the wagering requirements.
Most casinos forbid minimal risk betting strategies with credits, especially if these are part of a low-wagering bonus.
In this sense, an operator will invalidate your winnings if it discovers that you have employed prohibited gambling schemes, such as any bet spread combination covering more than 67% of the roulette table, for example.