Vip gold bonus was $100 for me as well, it is standart and reward doubles after gold rank. Regarding additional bonus, it is based on account's last 7 day activity if you lost more than 1% of total wager it gets bigger. I have seen bronze account gets $300 recent gameplay bonus, probably he lost all in single bet then got this rank.
Hmm your explanation seems more reasonable because I have also heard that Stake has implemented an extra bonus formula for those who lose more
At first I thought that the recent gameplay bonus was taken from the highest wins

therefore, when my progress leaves 1% then I increase the amount of my bet and hope to get something big and in fact this formula is wrong.
I prefer to be in profit and get less bonus than 10% of recent losses that been applied in many cases. Plinko is awesome option to wager and leave session in profit