Becuase the market price isn't $10. There is a price people are willing to buy and a price that people are willing to sell- which makes sense because if there's

Becuase the market price isn't $10. There is a price people are willing to buy and a price that people are willing to sell- which makes sense because if there's a price in the market someone is willing to sell at and I'm willing to buy at that price I won't put an order in the market at all - I'll just trade against their order. So there's a difference between the bid and the ask. So in the market you'll have people willing to buy at $9.90 and sell for $10. Now in reality the true value is somewhere in that range. But if I think the real value is $9.93 I might not want to put an offer in the market for $9.95 because the price might move, and if the price moves I'm stuck paying $9.95 for something that just dropped to $9.70. For the offers I put into the market, I have to add some extra margin to account for risk. Whereas if you come directly to me and say you want to $9.95 - well I know you're not going to move the market, so maybe I'm willing to trade with you for a smaller margin, safe in the knowledge I'm not taking on a very risky trade.