You're correct that Steem Dollars are supposed to be pegged one to one onto the US dollar, but it does fluctuate. Last I looked 1 SD is about 0.86 USD. So in this example, 10 SD = $8.60 and when converted to Steem valued at $0.60, would give 14.333 Steem.
The Steem Backed Dollar (or Steem Dollar) is it's own token that tries to hold a one to one value to the US dollar. This has it's own fluctuations of the exchange rate, lately one SD has been a bit under $1, but I've seen it over as well. So you basically have two exchange rates you're dealing with when converting SD to Steem..the SD/USD rate AND the USD/Steem rate.
Sorry if that was confusing. Feel free to join the #steemprentice channel in steemit.chat as there are people better suited than me who can answer questions. :)