No, that is correct.
When we started Dusty the price of STEEM and SBD were both around $3USD. At that point, an upvote worth $2 returned 1 SBD in profit. That's why we picked 2-for-1 for all transfers. As the price dropped, we have now a dynamic model where the amount subtracted from your balance is reflecting current market prices.

Check out the FAQ and the latest Dustsweeper post for a more detailed explanation.