Since Epic started taking a 12 percent cut of sales revenue generated on its new Games Store , much has been made of whether Steam's baseline 30 percent revenue cut is justified. But a new analysis shows that Valve sometimes receives much less than that headline revenue percentage for some of the most popular games on Steam.

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.

Valve doesn't directly publicize how many of a game's sales come from keys versus direct Steam purchases. But as Twitter user @RobotBrush recently pointed out, the Steam store does publish the numbers of user reviews that come from Steam purchases vs "Other" key-based sources (a feature designed to prevent key-based review manipulation).

Those review ratios might not match exactly with the overall purchase ratios—maybe key-based purchasers offer reviews at significantly different rates than direct purchasers, for instance. Still, it's the best publicly available estimate we have, especially when taken across multiple games. So over at the ResetEra forums, user Nappael did just that, using a bit of coding to pull out the Steam versus non-Steam review ratios for the 100 most popular games on the service (as defined by Steam Spy and a supporting API).

The results (for non-free-to-play titles) show that "on average, 72 percent of games are purchased through Steam, while 28 percent are purchased through third parties." And while that range can vary widely for individual games, the vast majority (80 percent) register between 50 and 80 percent of their reviews through direct Steam sales (You can view the raw, per-game numbers in this Google Doc).

When the revenue Valve is missing from these Steam keys is taken out, the company's average cut across these popular games hovers around 20 percent of all sales revenue rather than 30 percent (varying non-Steam sales prices complicate this calculation a bit, though Valve requires key-based sale prices to be comparable to prices offered on Steam itself). That's all before you consider Valve's December decision to lower its topline cut by five to ten percent for games starting when they hit $10 million in revenue.

Remember also that Valve incurs plenty of costs for these Steam key sales. Beyond the mere bandwidth costs for game and update downloads, key-based sales can still access the same online lobbies, achievement and leaderboard systems, Steam Workshop inventory management, Steam server APIs, anti-cheat services, and everything else that comes with being on Steam. Epic, on the other hand, provides very few of these services in exchange for the 12 percent cut on its Games Store (though the company does have a roadmap to roll out many similar features in the coming months).

Additionally, it's worth noting that while Valve is often taking less than 30 percent, this doesn't mean publishers are necessarily keeping more than 70 percent of their revenue by using Steam keys. Those keys are often sold on other platforms that take their own cut, which sometimes amount to the same as Steam's (though platforms like Itch.io and Humble Bundle generally take much less). Steam also imposes some limits on key generation to prevent developers from essentially piggybacking off of Steam's services while solely selling games directly to consumers elsewhere.

None of this necessarily means Steam's 30 percent cut of direct game sales is justified, or that Valve couldn't afford to lower its rates. But the next time you hear about Valve's 30 percent headline revenue cut, keep in mind that the functional proportion of total sales the company receives is probably quite a bit lower.