Why in male pattern baldness, hair at the back of the head and at sides doesn't affected?

[mminh]

I'm sorry if the question is stupid, but I try to find it but haven't got an answer. Maybe I use the wrong keyword. Anyone can help me please?

 

[RStGeorge]

Because those hairs are apparently not androgen sensitive.

Only hairs that are androgen sensitive miniaturise due to DHT, which comes from testosterone.