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.
It's vastly simpler, and more useful, for Steam to detect the current resolution. Trying to detect the maximum supported resolution is non-trivial, especially when there are devices that will accept a 4k signal despite having fewer pixels.