When you add more reactant, the equilibrium is disrupted, over a period of time the concentrations of the species in the chemical equation will return to a state such that the value of the expression for Kc is once again Kc. Ie. in real life after adding species to mixture, it is of course not in equilibrium immediately afterwards.
The reason that the the relative concentrations can change such that the proportions are not the same as in the original state is that Kc is non linear and often dependent on more than just a single product and reactant.