Question about Dell XPS 15z resolution

Dear all,

How are you?

I hope that every one is doing well. I have one issue in the resolution for Dell XPS 15z.

When I set the resolution, it gave me some options like (1920, 1080). If I choose this option and then I try to get the system resolution using programming language for example, does not give this one. It actually gives the below resolution: (1280, 720). Does any one know why it is like this? What is the relation between the given resolution and the printed one.

Thank you and regards.

BR//