I am running a 4k display but Speccy is reporting resolution as 1920x1080.
It seems to get the monitor width and height correct (3840, 2160).
BTW - this is Speccy v 1.31.732 (64-bit)
Not sure if relevant - I am running Windows10 in a KVM passing through the USB and video card using vt-d. Was curious what the "machine" looked like to the VM, and this information was interesting.
See screenshots below.
(Thanks for an excellent tool!)