-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
I got it twice in the past , once around 300$ and once 500$, guess the 500$ was a bad month for me. Been a while though.
I don't think the bonus is group based. They just take the top players from the site and give the bonus. If it was group based then this would be a little unfair if for example you are in a group/with a host that has a lot of super high rollers.
That would be too unlucky. I know the VIP host writes something like "you have been one of my best players" which indicates it's group based but I think they just write it that way. It sounds better.

I think it is in this format aswell. They must split it up amongst other players whom qualified for the bonus.
During one chat session last month, I remember them saying they received $800 for their top monthly but had lost upwards to the amount of $10k to reach it. Which is a massive amount so they did get some of it back in this way.
As to @ryzaadit question on how much was it I received, it is the same as the above has explained. $250-300 in the coin of our choice.