Dear Madam,
A genuine and very interesting question posed by you. I will love to address this doubt in a unique manner.
Actually the way you have posed the problem holds the key. The residence time you have mentioned are not sacrosanct and varies with many causal factors which you will come to know during the advanced training sequel of this basic training wherein meteorology will be discussed in detail. Nevertheless both meteorology and hydrology are inexact sciences, and as on date these are not captured 100%. Many parameters are responsible for the uncertain variability like complex interaction of land and water, energy flux between earth and space, macro and micro events, lack of precise and full data set, inevitable changes of land use and climate, impact of the resultant effect on the causal agent itself, computation capability, etc etc to name a few.
Even in a wet year, rain is not same in all the days, not same in all the hours, so infinite difference and variation is integral part of nature and we feel change because of this. Might sound philosophical but its a deeper understanding which will lead us to this conclusion. Space and time are infinitely differentiated and nothing will be same. Only our understanding can bridge the gap of uncertainty and ignorance. Think of system concept and you will realize that the system boundary and environment are constantly interacting, flux. The first question placed by Dr Purandare if you recall, interaction of SW and GW is an apt example for this. You will never be able to catch the exchange of water 100% because soil, rock, energy, pressure, pores, mass of water, vegetation/crop etc are not uniformly distributed. Modeling can help us to a greater extent and more with advancements in instruments, technology, understanding, science, etc to the extent humans need to know for their benefit. Hope i am able to throw some light in an abstract manner.
Thanks for thought provoking query!!