I guess I think of it in a very linear way. To get the result I want, I'd have to sort by author first, then within each author, sort by title - that's the order in which those steps need to be taken in order to get the result; if you sorted by title first, you wouldn't get the right result. Or sort by read/unread first. Then within read and unread, sort by author. Which could be re-cast as by-author-within-read-status - but I still think of read status as being the first sort that has to happen.
I do see your point - after all, I've been doing it in calibre without thinking about it for a long time now: I click read status last, because that's ultimately what I want to see. But if I do think about it, the way calibre does it makes me confused, because it's not the order of the steps that have to happen, as I see it.
But yeah, brains are strange!
(And I guess it's been too long since I've used Excel - I was projecting!)