Thank you for this explanation!
I wondered what had gone wrong with my recent conversion.... I wish I had known that the "convert to steem" option in the wallet next to my SBD in essence does just this:

1 SBD entitles me to $1 of STEEM by design. Therefore, 15 SBD entitles me to $15 worth of STEEM. Using the price of $1.70, I normally would get 8.823 STEEM ($15 divided by $1.70).

Whereas if I had gone through the marketplace I would have gotten:

I received 104.272 STEEM which is 11.818 times what I normally would get.

Lesson learned!