Explain why all cells in a population do not die instantaneously when exposed to an antimicrobial agent, even when they are of a single species.

The killing of the microorganisms does not take place instantaneously when getting exposed to an antimicrobial agent or a lethal agent, however, the population gets reduced by a constant fraction at constant time duration, known as exponential killing.  

The microbes react with their surroundings, making modifications in their internal environment. Also, there is a certain threshold of an antimicrobial agent, which must be met in order to kill the entire population. The cells, which are active get to die or killed briskly as antimicrobial agents target the metabolic procedures of the cell.