Aside from other sins of the BMI index, the weight of the words we use to classify BMI are absurd:
"Under" means shouldn't be this weight, so does "over." "Normal" implies everyone should be in this range. All this is ridiculous. BMI was never meant to assess individuals, anyway, it's a population measure.
But making quantitative and value-laden classifications that indicates someone ought not be in that grouping is bad science
First of all, weight is influenced by many factors -- mostly genetics -- and isn't something one can decide to simply change.
Think of it this way: let's measure everyone's lung capacity and graph it up. There'd be people with readings all over the range, and some on the far edge are very unwell. But others with low capacity may be quite well, and may indeed be doing as well as they possibly could be considering their genetics and medical history. Those people "should" not be considered "under-breathed.
There are many, but here's yet another reason to look with derision on the "overweight" wording: