Two monitors connected to PC but not running smoothly?

I have two screens and a PC with a GTX 1080 TI, but when I eg. On a fortnite game and on the other youtube watch one drops where Fortnite runs on 60 frames although it should run 144 fps, but it only looks 60 fps but 240 fps above because that's the maximun?

Why is that?

Ever measured the FPS with the tool, it sounds as if you have estimated the FPS

Well you can see very strongly that it is no longer 144hz but less