Because your monitor is (probably) refreshing at 60hz. Your frames will display for some integer number of monitor refreshes - at 1 you'll get 60fps, at 2 you'

Because your monitor is (probably) refreshing at 60hz. Your frames will display for some integer number of monitor refreshes - at 1 you'll get 60fps, at 2 you'll get 30fps. Over a second, it's of course entirely possible to get something in-between, where some frames are displayed for 1 refresh, and others for 2 refreshes. Inconsistently waffling between these is going to be more jarring than sticking to one. Recently, you have things like nVidia's... gsync? Which make the monitor refresh rate variable with compatible monitors, to match the game framerate, making 45fps entirely possible, and reducing the jarring from varying frame lengths. But this recent, rare, and likely PC and/or GPU vendor specific still.