Why do I have fps drops with every Fps cap?

When I cappe my fps to 160 in Fortnite, my fps drop to around 100fps. If I play at 120 they drop to 80 and at 60 to 30. What can be the reason? And which cap should I use best?

Then just don't use a cap at all?

Do you have Vsync on?

No

Yes, but I'm always advised not to play indefinitely because it ironically causes fps drops