Does anyone know why mature hairlines happen and what causes them?

My father has had a slightly receded nw1.5 hairline since 25, for the next 30 years it stayed this way. and only now at 55 is he receding a little bit further at the front and very slightly diffuse thinning

How can he stop balding at 25 at a nw1.5 and start to bald further at the front and in a diffuse manner when his dht levels are lower than ever before?

It doesn’t make any sense in my opinion.