Actually not, the rewards should be based on the STUs system, but sideways get affected by the USD steem price. I would look for a better explanation.

Found this on a steem post via google;

As for why the dollar value of the vote tally varies from minute to minute it is not that complicated.
The value of each vote is dependent on how many people vote each day and the price of Steem.
This is because the amount of Steem produced each day is fixed and hence a fixed amount of this is available as payment for posts via voting.
The values are only estimates and are likely to vary over time based on the difference between the number of people actually voting and the prediction system which determines the value of the vote.
In addition to this the actually moment to moment value of Steem will also vary which will also affect the figure.
To be honest given all the variables it surprising that it does not change more often

I would add that since a portion of your upvote value goes as SP, the lower the price of the Steem is vs. USD, the more SP you get via rewards in SP, but it should be leveraged by lowering your SP upvote value, this is, the indirect amount of SP you get as curation.

I guess is just a way to avoid people getting thousands of SP if the price sinks all of a sudden.