Unless a sample of particles is perfectly mono-dispersed, i.e. every single particle has exactly the same dimensions, it will consist of a statistical distribution of particles of different sizes. It is common practice to represent this distribution in the form of either a frequency distribution curve or a cumulative (undersize) distribution curve.
The video below discusses the use of these distribution curves.
The graph below shows a frequency distribution curve for a bi-modal distribution of particles. The distribution of each mode can be varied to examine its effect on the typically used particle means.