Tracking the dollar would mean that someone would always be willing to buy your SD 1:1 for dollars (via steem). This is obviously impossible. Steem tries to have its value as stable as possible by forcing users to lock their steem in the system (via power ups). This, combined with the general consensus that the steemit platform is promising keeps volatility very low. But in no way does this mean that SD and $ is of equal value.

As I said, the only way to guarantee this would be the steem organization buying back steem at a constant price, which is something they would never do. Pegged currencies never worked anyway. Look at dollar to peso, dollar to gold etc.