I think I’ve read an explanation for this before, but I can’t remember it and can’t seem to find it.
It seems that being slightly overweight is good for your lifespan. So why is this part of the weight range classified as “overweight.” That implies a problem, which doesn’t seem to manifest.
Why not change the classifications?
All I know is that according to the BMI chart, I need to grow half a foot.
I didn’t know this. As one who is stuck on 26, I’m happy to hear it and looking forward to pointing it out to my doctor. 
Something wrong with the two you have?
Guess: Because the average BMI category includes people who have lost weight because they’re sick.
We’ve done this many times and each time the doc’s tell us it’s pretty much this. Many people in their last months of life are pretty scrawny due to disease; they skew the survival statistics down for thin people and make the mildly overweight people’s survival rates look better.