I also notice that there is a preference for low prices (a 1.1 bid and a 0.9 ask leads to a 0.9 transaction price, not 1)
This has the advantage that you can easily buy a large quantity without needing to enter lots of different bids.Suppose there are asks at 0.6, 0.7, 0.8 and 0.9, each offering to sell 10 BTC. You want to buy 50 BTC, and each BTC is worth 0.95 to you. You simply bid for 50 BTC at 0.95, and the system matches them up: you buy 10 at 0.6, 10 at 0.7, 10 at 0.8, 10 at 0.9, and your bid for the remaining ten at 0.95 stays on the order book.
If it didn't work that way, you'd end up paying 0.95 for all of the coins that you buy. No-one would want that, so they would need to enter separate bids at 0.6, 0.7, 0.8, 0.9 and 0.95.
