Earlier this week, Sony celebrated a (somewhat arbitrary) milestone for PSVR: 4.2 million headsets sold. It sure sounds like a lot, but if you put it in comparison to other excessively publicized console accessories, like the Sega CD or the Kinect motion camera for the Xbox 360, it’s a far less impressive achievement, as noted today by Ars Technica.

There are two important numbers to look at for the PSVR (or any accessory): how many units it’s sold, and what percentage of total console owners have bought it, otherwise known as the attach rate. In the PSVR’s case, Sony has already told us that sales have reached 4.2 million, compared to the 94.2 million PS4 consoles sold as of February 2019. That translates to roughly 4.4 percent of PS4 owners having bought a PSVR headset since launch.

But compared to other console and accessory matchups, the PSVR is doing worse. Consider the Sega CD, which Ars Technica notes sold 2.24 million units to roughly 30.75 million Sega Genesis owners, or about half as many PSVRs. Proportionally, Sega was able to get nearly twice the number of total Sega CD owners to buy in, compared to Sony’s VR headset, with 7.2 percent of Sega Genesis owners eventually buying the CD attachment.

Comparable to the Sega CD, but not the Kinect

Things look even worse when you consider the original Kinect, which was by most standards a failure for Microsoft. Both the original Xbox 360 version and the later Xbox One Kinect model failed to break through to the mainstream and widely popularize camera-based motion gaming. Microsoft no longer even sells the Kinect, and current Xbox One S and Xbox One X consoles don’t support it. But the company did manage to sell a ton of Kinects to people anyway: Ars Technica’s report cites 24 million units sold to a total of 76 million Xbox 360 customers, meaning that nearly one in every three Xbox 360s had a Kinect.

It’s not just a popularity contest, either — there are very real consequences for how well these accessories sell, especially when they enable new types of gameplay or wholly new games. Developers ideally want to sell games to the widest audience, and if only one-third of your customer base has a Kinect (or in Sony’s case, less than one-tenth of the console base owning your VR headset), a developer is presumably much less likely to create a game that exclusively works with it.

It’s one of the reasons why Microsoft bundled a Kinect with the Xbox One at first. The goal was to make sure all new console owners would have a Kinect, and that way developers would have a built-in and relatively sizable audience for which to build motion-sensing games. Again, this idea failed both because Kinect games never really caught on and also because bundling the second-generation version with the Xbox One device made the package more expensive than the competing PS4, leading to the eventual unbundling of the two devices and the killing of the Kinect ecosystem. But hey, Microsoft at least had a sound strategy in mind. (A lot of the technology inside the Kinect eventually made its way to the HoloLens, so it wasn’t a total wash.)

While the PSVR’s sales are certainly impressive, especially compared to other VR headsets, it’s ultimately still a very small fish in a very large pond of traditional consoles, and even other accessories. And barring a major shift in VR tech or strategy on Sony’s part, odds are that fact won’t be changing radically anytime in this console generation.