为什么SBD会与美元脱锚？Why is there such a big price fluctuation?

The primary concern of Steem feed producers is to maintain a stable one-to-one conversion between SBD and the USD.
"Stability is an important feature of successful global economies. Without stability, individuals across the world could not have low cognitive costs while engaging in commerce and savings. Because stability is an important feature of successful economies, Steem Dollars were designed as an attempt to bring stability to the world of cryptocurrency and to the individuals who use the Steem network."
According to recent the market feedback, it doesn't work very well. If the primary concern of SBD is to maintain a stable exchange rate, and the independent monetary policy can be abandoned if necessary, why is there such a big price fluctuation? Is there restriction for capital flow? Or steem still keep an independent monetary policy?
SBD的首要目标是维持币值稳定，这在steem的白皮书中也用了一段话进行了明确的表示。但从市场近期的反馈来看，这个方案貌似并不成功。如果首要目标是维持币值稳定，并且可以放弃独立的货币政策，为什么SBD还有如此大的波动？是因为限制了SBD的自由流动还是steem仍然保持了独立的货币政策？