I think it's to do with the spine compressing as we age.
My grandmother was always a lot shorter than me, and I recall fondly an elderly gent telling me my grandmother was such a beautiful woman, so tall end elegent. I was also confused by it though as she wasn't tall. So I asked my aunt who said, yes she was about 5ft 9. I'm five eight.
When she died and they laid her out effectively she regained most of the lost inches as basically they stretch the body straight. My aunt remembered my comment and said, you can see how tall she was now with a sad smile.
Anyways, I say I'm five eight, I was five eight, I think I might be shorter now and more five seven and I'm 49. I'm not sure and would need to be measured again. The thing is I have a short body and long legs, so I don't need my body to shrink any more as I will just look even weirder than normal 🤣