If the radiation is especially intense, nonlinear optical processes can also cause variances. The main reason, however, is that the concentration dependence is in general non-linear and Beer's law is valid only under certain conditions as shown by derivation below. For strong oscillators and at high concentrations the deviations are stronger.
When radiation intensity is very high, nonlinear optical processes can lead to variations in results. The main reason for this is that the relationship between concentration and absorption is non-linear, and Beer's law only applies under certain conditions as shown by a derivation below.  Strong oscillators and high concentrations result in more significant deviations from this law.