This is pure speculation. I think that the bitcoins are only put there as an IOU. No real bitcoins are transferred anywhere until the escrow is paid out. That's when the everything happens. So, when it gives you a dollar amount its just that an amount that is locked in. Because escrow isn't done til the transaction is done, the amount in bitcoins could go up or down..At least, that's my take on it... So, all the bitcoins are just stored in a secure place. No matter how you get your bitcoins on SR I believe it always go through the tumbler somehow...(At least that is my theory and my hope)..