At the moment SD is converted on a "7" day average, by using the "Convert to STEEM" command, it takes the average price of STEEM based on the last 7 day market feeds, and gives you the equivalent of $1.00 worth of steem for that week.  This can happen without using the internal market.  If you do use the internal market, then you should be prepared to research if you're buying at the bottom, or the top prior to deciding on what price you are selling it for to ensure you are getting either "more" or "less" than the average $1.00 worth of steem.