X will always be less than 10DRK, therefore we need another input to bring it to 10DRK total. This way no one can tell who is receiving which inputs, thus cleaning them in the inverse way DarkSend works. Checkout my post above again, I modified it a bit.
Make sense?
If X1 is just some other change address that belongs to John then yes, I got it. I was expecting X + Y = 10 DRK, introducing X1 made it ten times more confusing for me.