I think you are confused. There are two completely separate prices:

Price 1: The internal market price, set by the buying and selling of STEEM/SBD.
Price 2: The internal blockchain price, set by the witnesses price feeds.

There is also a difference between "converting" SBD to STEEM (done through the wallet, and takes 3.5 days), and "trading/exchanging" SBD for STEEM (done through the internal market, or through an external exchange).