There are two reasons we specify that it is an average. First, individual atoms move around due to thermal motion, and the distance between a particular absorbing atom and a particular scattering atom varies a bit. This is called thermal disorder. But it is also true that not all absorberscatterer pairs represented by the path are necessarily identical. There may, for instance, be defects such as vacancies or dopants in the material. There may also be phase boundaries or surfaces. In addition, you might be modeling a distorted shape as if it were more symmetrical, perhaps choosing not to make a distinction between equatorial and axial atoms. These are examples of static disorder.