Event System value of mouse position is wrong.

I've just never had this before, and I'm not sure what to do about it, google didn't really help. Basically as I show on the video, I have a button with 920 width (which is important) in the middle of the screen. Since Game view is full HD (1920 width), there's 500 padding on left and right. When I hover over top left corner of the button, it's on around 480px, not 500px, and everything to the left of 500px is not responsive. Any suggestions on why is this happening and how can I get rid of it ?

https://youtu.be/Ga_nPTXKD6w for the reference.

If replicated without a single line of code, with a Button on a Canvas and an Event System, problem still persists.