I'm surprised 4K monitor adoption is still under 4%, less than 1366x768 laptop resolution. I guess I was blinded by graphics card marketing, most can't really afford to shell out for 4K yet.
OP is showing the multi monitor setup. (i.e. users like yourself that have two or more monitors). There's a separate tab that shows single monitors. That one does have 1920x1080.
Probably based on what the actual monitor resolutions are and not just the raw multi monitor result as you can create an almost endless variation of those when you start matching the resolution position based on the physical location of the monitors.
E.g in a setup like this even if both would be 1920x1080, you'd end up with something random like 3840 x 1428.