Every metal has a definate work function. Why do all photoelectrons not come out with the same energy if incident radiation is monochromatic?

Explanation

Work function of a metal is the minimum energy required to knock out an electron from the highest filled level of the conductionband. The conduction band comprises of different energy levels forming a continuous band of levels.
Electrons in different energy levels need different energy for emission. Thus, the electron, once emitted have different kinetic energies depending on the energy supplied to the emitter.