I suspect it has something to do with physical activity level, based on a single piece of anecdata:

Starting at age 30, I was a distance swimmer, either in the pool or at meets. Two miles in the pool five or six days a week. My temp was usually 99 or higher. I ate like a teenager the whole time and never gained weight. I was usually hot when everyone else (coworkers, my wife) complained that it was too cool. Note that this is a higher-than-usual-today activity level.

I then got a brain infection from a dental cleaning, came very close to dying (brain surgeon and infectious disease specialist both said the only reason I survived is that I was ‘very fit’). Lost 60+ lbs. Had to re-learn walking. And my usual temp is down to 97-98.