Is an overweight but not obese BMI protective, especially as you age?

There is a JAMA study from 2013 (see below) that seems to think it might be.  The study is a meta analysis that compares the hazard ratio of overweight BMI (25-30), and obese BMI (>30) to normal BMI.  Obese BMI was more hazardous than normal weight, which is what you would expect, but overweight was less hazardous, which is somewhat surprising.  

The effect is even greater for people over 65.

I checked that "normal weight" didn't include underweight people, and I don't think it does.  

 Flegal, K. M., Kit, B. K., Orpana, H., & Graubard, B. I. (2013). Association of all-cause mortality with overweight and obesity using standard body mass index categories: a systematic review and meta-analysis. JAMA, 309(1), 71-82