The 231 days gap is a big factor. Some airlines will want their planes sooner rather than later, which means the first one to the market will have an advantage. There is more than 231 days of gap between their entries into service. So if airlines are ordering planes out to, say, 2020, then naturally the type introduced earlier will have more orders. A graph from the linked source shows the number of orders based on days since launch. The A320 still comes out ahead of the 737, but the difference is not as dramatic.
Although the list prices are similar, no one will actually pay those prices. Both Boeing and Airbus will negotiate heavily with their customers to secure sales at much lower prices. The prices actually agreed to are typically not published, but it's possible that one factor is that Airbus is willing to give more of a discount.
Features are also not really identical. They may be very similar on the outside, but each manufacturer will tell you about the features that distinguish their product from the competition.
Another important factor is where the orders are coming from. Asia is a rapidly growing market, and the map shows that Airbus has received far more orders there. Airbus has an assembly line in China, which could be another factor working in their favor.