Lots of products, not just games are prices at X9.99. It gives the consumer the illusion that the price is actually cheaper than it really is.
For example, a person may look at a product that is $49.99. And appears to be cheaper than if it was $50 outright, as the first digit is often the dominant digit and retains in the consumer's head. The product is actually cheaper priced at $49.99, but by an amount so miniscule it's usually not worth worrying about.
I hope that explained something.