Fitness is better assessed by the rate at which you recover after doing something physically and mentally demanding beyond your usual abilities.
Fitness measurements, assessments, Formulas and tests are guidelines and estimations to where you are not real indicators of your general health or abilities. For example the BMI is a standard that gives a rough idea of how you are in relation to your height and weight compared to others. There are many natural variance as diverse as humans are! Everyone is unique and more specific measures likes muscle mass, bone mass or body fat, V02 Max of % of bodyfat measurements gives a more clearer indication of where you are and what needs to be done.