The thing is that SBD is half pegged. There is some sort of limited supply and the result is that if there is more demand than supply the price will go up. If the price were to go below one USD things get messed up. You would then be able to buy steem (which has it price in USD) with SBD which would mean profit by doing so. Just and example. Suppose SBD is at 0.5 USD. You can now buy 2 SBD for one USD. With those 2 SBD you can buy one Steem. You can now sell that one Steem for 2 USD. With those 2 USD you can now buy 4 SBD, .... This actually happened in the past. Steem witnesses can adjust some parameters of the system, inflation, to combat this.
So in one direction action is taken, in the other direction not. Hence half pegged.
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.