This assumption may be false; the risk is that we ignore the possibility that the quantity is indeed dynamic and that the variation we observe is intrinsic to the phenomenon. Naturally, we prefer the simpler model of a stable quantity when it is tenable, but we must examine the phenomenon to see if it warrants such a simplification.
We may observe distinct members of a population which, ideally, we would like to consider interchangeable. Only in the arena of manufactured products do we approximate such a situation, and even there, the meticulous scientist finds idiosyncracy of sampled items. Hence, the differences between individual units becomes part of statistical variability.