Websites are telling my i'm 2560x1440 when I'm 4k native and set to 3840x2160, why?

[Rossdowling321]

http://www.screenresolution.org/

and

http://www.whatismyscreenresolution.com/

are my sources, they say the correct resolution on my 1080p monitor, but not on my 4k one? why is that? I have a 4k 60hz dp 1.2 cable and the monitor is a 4k2k 2160p 60hz ips monitor.

Thanks in advance.

edit: I have a B276HK Acer 27"

 

[Rossdowling321]

I found the answer to my problem, I have windows/apps/text increased from 100% to 150% so things are readable etc, this also makes software think that is your screen resolution, which in fact it isn't, so it reducing it to 100% brought all the tests back to my native which was 3840x2160.