The 7 day conversion comes with a certain risk, which is priced in on the SBD market. If you convert, you'll be subjected to Steem price changes in that period. Seems like the market expects Steem to go down.
0.85 cents is a very big spread though, Steem would have to drop ~30% in the next week for conversions to be a losing decision.
RE: SBD isn't broke: How SBD works on STEEMit