Yes, I have seen it in reverse, but was not so extreme (was a few months ago). But if SBD = 0.5 USD and, in your example, 1 STEEM = 2 USD, then the market would notice the trades in selling STEEM to buy SBD, thereby balancing the prices.
Perhaps the markets are not yet very efficient - less so in the early days. I mentioned the 2-week lag in the STEEM/SBD price movement. The trade volume within the Steemit internal market appears small compared to the external exchangers, and speculators may not all realise the relationships between the two currencies, hence the adjustment took longer than I expected at the time.