Valve allows developers to generate Steam keys to sell through any other platform they want for any price they want. Not only do they do that but they don't take a cut of what those keys are sold for.
The reason for the discrepancy is Steam keys, which developers can generate pretty much at will to sell through non-Steam storefronts and brick-and-mortar retailers. While these key-based purchases are still redeemed through Steam and can take advantage of Steam's suite of features, Valve actually takes no commission from sales that don't take place directly through its own storefront.