Statistical variability is quantifiable dispersion of measurements of differing members of a population within the scale on which they are measured.
One of the forms in which statistical variability is realized in the emprical sciences is that of differences in repeated measurements of the same quantity.
In the physical sciences, such variability may result only from random measurement errors: instrument measurements are often not perfectly precise and accurate. One may assume that the quantity being measured is unchanging and stable, and that the variation between measurements is due to observational error.
In the biological sciences, this assumption is false: the variation observed might be intrinsic to the phenomenon: distinct members of a population differ greatly. This is also seen in the arena of manufactured products; even there, the meticulous scientist finds idiosyncracy of sampled items.
The simple model of a stable quantity is preferred when it is tenable. Each phenomenon must be examined to see if it warrants such a simplification.