Unfortunately, desktop hardware is not designed for "real time" applications, where "real time" is defined as "thing X has to happen exactly at N".
As computers get faster, we're edging closer to that goal, but at the same time the devices try to do more and more things at the same time.
In other words - while your graphics hardware may be capable of rendering whatever you're doing at smooth 60hz, nothing stops the operating system from deciding, all of a sudden, that you have to start downloading patches to the instant messenger you've never even launched, which in turn requires the system to spin up the hard drive that was put to sleep some time ago, which again may cause some interrupt request noise and ends up causing you to miss a frame at some border case..
So what I'm trying to say is, this has little to do with GLUT or GLFW (although you should never use GLUT for anything serious), and the problem is of a bigger scale. Maybe if you build a bare-bones linux distribution that will never, ever do anything behind the scenes..
Or you can just live with the fact that you may experience some rare frame jitters.