Consider an experiment in which an electromagnetic wave whose polarization is along an angle $a$ with the $x$ axis is sent through a polarizer whose polarizing direction is along $y$ axis. The probability that light will be transmitted is $\cos^2 a$, which is the ratio of transmitted and bombarded intensities.
Is this probability absolute? Because when we always measure classically the transmitted light the intensity ratio is $\cos^2 a$. Suppose we send in 1000 photons. The probability to get absorbed or transmitted is $50:50$. Does accurately 500 electrons will get transmitted or this number can vary to an extent. But if this varies why we always get intensity ratio equal to $\cos^2 a$.