-snip-
That's just based my speculation.
Could be each rank have different budget you can receive for top monthly wagering. Imagine, you're "Diamond" rank and only get 200-300$ can't really believe for player who can waggering at least more than 500K or 1M.
But they get only 200-300$ ~XD
Sure, I'm speculating as well because there is no exact information about this and other bonuses in stake except the level up reward.
However the bonus might be the same as how Stake calculate the monthly bonus where it is based on several things such as rank, wagering statistic and profit statistic.
For additional information, I received them when I was Platinum 3 and my wagering statistic when I received the Top monthly bonus were around $300k per month.