That i can sort of understand as the $ value is based on the amount of SBD you have and the exchange rate between SBD and $ fluctuates, but to devalue by 20% seems quite a lot. So the other reason is probably the $200 was based on when you first posted something on day one you get a bigger slice of the reward pot, but by day seven more people have posted as well and they get more upvotes than you which eats into your share because there's just one fixed pot.