All the voices in my head

How is it that whenever I read a post or an article, the inner voice that I use to read silently automatically creates a character to fit what I imagine the author to sound like?

For example, it goes without saying that if I read a post written by a known female, my silent reading voice switches to that of a female as if I am imagining what she would sound like according to any if at all anything I may know about them. A male author switches this inner voice to a male character.

I have noticed that if on occasion I read something written by a child, the inner voice switches to that of a child. The more I know about someone without actually meeting them changes the voice.

If I read someone's stuff and they generally tend to be aggressive, this changes the voice to a more aggressive tone.

Why is that?