Why is the mechanical advantage of a simple machine always less than the calculated value?

Hint: Mechanical advantage is the advantage achieved by using a mechanism to transfer force, more precisely the ratio of the force that performs the useful work of a machine to the force that is applied to the machine. Because friction exists in all real devices, the output energy is always less than the input energy.

Complete Step By Step Answer:
Because no mechanical system is completely efficient, the real mechanical advantage of any mechanical system will always be smaller than the ideal value. Real-world losses occur throughout the operation of even the most basic mechanical systems, and are reflected in the real forces necessary to run the machine and are not accounted for in "ideal" estimates. These losses can be vast and varied - far too numerous to enumerate - but they are most commonly linked with heat loss owing to friction and forces absorbed via elastic deflection.
Mechanical advantage is given by the equation
$MA = \dfrac{{{F_o}}}{{{F_i}}}$
Where MA is a mechanical advantage, ${F_o}$ is the output force, ${F_i}$ is the input force.
Mechanical advantage is a measure of how much your force is multiplied, whereas efficiency is a measure of how much of your work really goes toward moving the item you're attempting to move. For example Increasing the number of pulleys improves the mechanical advantage, but it also increases the amount of extra stuff you're moving.

Note:
The efficiency of every realistic machine is always less than one. We can make the efficiency almost equal to one by employing superior manufacturing technologies and high quality and appropriate lubricants.