Not all gamers have a computer entirely dedicated to that purpose. Even among those that do, it's not uncommon to also play games or run Steam on another machine.
I still have Steam installed on the laptop that was long ago replaced as my gaming computer but which is occasionally used for other purposes, because I have no particular reason to remove it.
It's actually probably reporting the software-configured resolution, not the hardware capability. The important distinction is whether it's a system-wide resolution setting or a game-specific setting that may not apply to browser contexts (except for the ones used by Steam itself).
What makes you think that it’s more likely reporting a software-configured resolution?
It is after all a hardware survey, and focused on what user hardware supports.
