Question

Why mechanical advantage is always less than the velocity ratio of a actual machine

Answer 1

MA advantage is always less than VR for an actual machine.
Reason: For an ideal machine, it's MA and VR remains same. But in an actual machine, always some input work is "wasted" as heat energy, sound energy, etc. Due to this waste only there is the MA advantage is lesser than what it was in ideal machine. Now taking about VR; it is the ratio of displacement of effort to the displacement of load. For a machine of given design, it's VR remains constant whether it is actual or ideal case as VR depends on displacement of load and effort. For a machine, it will create a same displacement of load and effort both in ideal and actual case. But MA depends on load it can overcome by applying effort. And due to waste of energy, we get work output always less than work input.
THUS, we can conclude that MA for an actual machine is less than MA of same machine in ideal case. But VR remains same, both in actual and ideal case. That's why mechanical advantage is always less than the velocity ratio of a actual machine.

Answer 2

it is becoz in actual machine there is 
1. friction
2. string is not perfectly elastic
3. its diff parts are not perfectly rigid