Although at first sight this might seem to be biology rather than physics, maintenance of human posture is an example of control theory. Indeed a quick Google will find many publications on it - I found a good review here.
A standing human is a dynamical system that is not in equilibrium so it requires a (closed loop) feedback system to maintain it. This means sensors in your body detect deviations from your desired posture and muscles contract to return you to it. The problem is that there is a lower limit to the movements you can detect i.e. you body moves a certain distance before your muscles respond and move it back. The result is that even when we try to stand absolutely still we are actually continually moving then correcting that movement.
Some people are better at maintaining posture than others, so some people can stand stiller than others. Plus the ability declines with age, as I can testify from personal experience :-)