Suppose (for simplicity) there is an ask offer on the market 10 bitUSD @ 0.5 BTS/bitUSD and a bid for 5 bitUSD @ 1 BTS/bitUSD.
The market can choose:
1. take the 10 bitUSD from the ask, use 5 bitUSD to fill the bid, take the 5*1 BTS from the bid and pay the ask 10*0.5 BTS, keep the remaining 5 bitUSD for yield.
2. take 5 bitUSD from the ask, use these to fill the bid, take 5*1 BTS from the bid, pay the ask 5*0.5 BTS and destroy the remaining 2.5 BTS as dividends.
3. Anything in between.