I believe that fat redistributes differently as you age.
It's the only thing that makes since out of being 152 lbs at age 30 and 178 lbs at age 37 and having the same measurements & wearing the same sizes.
Whenever I'd gain weight, I used to always gain it in my hips & thighs. That was my 20's/30's. Now that I'm in my 40's, I still gain it in my hips & thighs, but also I find weight creeping into my upper abs and my arms & OMG... that horrendous BACK/BRA fat!!!! Ick. "Sometimes it's hardddddd to beeeeee....a womannnnnnnn."