I guess my brain thinks about sorting in a visual way
Title
..Author
doesn't do it for me... versus
Author
..Title
If I was describing this to someone, I would say "sort by Author then by Title". I would never say "Title within Author". You can get all temporal on me but perhaps because of my visual offset it just works for me
Like I said in my original post - if I'm just alone and weird in thinking this way then I will happily change it.