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 for @ryzaadit question on how much was it I received, it had been same as the above users have explained:
$250-300 in the coin of our choice and nothing more than this even with my losses for the month. I am minus in profit of nearly $2k.
Group based could work for the lower end, like anyone who wagered 1000 dollars or less that month, that would be very low number and you can group them all together, I do not see any problem grouping up people who wagered 750 bucks and 50 bucks all month long, they all feel like that would be similar. I can also see like 5-10 different groups under 100k as well, people from 1k to 10k, 10k to 20k etc etc, all those could make sense as well.
However, when we are talking about the very very top, like the people who wagered the most, even first would have a difference from the second. This is why grouping people together would work at the bottom, but the higher you go the harder it would become because there are people who gamble millions and millions every month.