The top seller bit is actually really easy to explain: StudioFOW is fully aware of how to exploit steam launches. Game visibility is affected on day one by pre-release wishlisting (and subsequently by sales and rating), and this game's page has been available, popular, and heavily wishlisted for something in range of an entire year. That volume of wishlisting and page views is pretty much guaranteed to catapult any game up to slot 1, so long as it doesn't need to compete with another launch using the same tactic on the same day.