Thus, the thing that is the model varies with the system under study, but not with anything else.

因此，模型这种东西是随所研究的系统而变化的，不是随着其他的而变化。

youdao

If your weight is excessive then the implication is that the excess is fat and, of course, people's body composition varies enormously and BMI doesn't know anything about that so it's a very imprecise index when applied to individuals.

BBC: BMI: Does the Body Mass Index need fixing?