Well... actually the vested STEEM rewards haven't changed at all, it is the amount of SBD that has reduced on your rewards.

Technically an identical vote (same amount of RShares) will result in the same amount of vested STEEM rewards (ignoring reward-pool fluctuations).

But since the whole "on-chain-conversion" happens on the assumption that 1 SBD = 1 USD in STEEM, the declining price of STEEM on the markets reduces the amount of SBD being created, this is also reflected on the $-reward-preview on posts. This number, too, is calculated under the 1 USD assumption.