I move sbd out of here frequently. I use blocktrades and the price is never what is says it is on any exchange. I check several places each time. It is always less when I transfer by a good margin. I used bittrex one time and got a slightly better rate, but not by much. I am not sure why this is, but it has been consistent since I started taking it in January this year.
I hope you get some help and understanding on this issue, as I would love to know the answer.