display Why does my monitor say it's at 60hz but I can tell it isn't

[Don_Don]

So usually, my monitor works fine, it's a 4k Dell P2815Q, but I scale the resolution to 1080p so that I can run it at 60hz. I have it hooked up to a KVM switch to use my mac which uses the monitor at 4k, sometimes when I switch from my mac to my desktop, even though my monitor says I'm running at 60hz at 1080p I can tell that it just looks choppier than it did before and also when I get on full screen games the hz changes to 30 and I don't know why. I've been playing around with the Nvidia settings trying to find a solution but I couldn't find anything, hoping someone could help me.