Your profit from auction is made by taking part of the settlement fund. For instance, you are bidding 100 BTS, and if success, you'll have a margin position with 175 BTS as a collateral. You got these 75 BTS as a bonus.
Apart from this, you are taking risk of further BTS price drop and be margin called loosing both your own 100 BTS and 75 "profit" BTS.
RE: How Global Settlements work