A combination of legacy and practicality.
The one in the books has always been there.
The one in the authors table is newer, and is the default value applied to new books. However you can over-ride the value at the book level if you want to. This tends to be of most use in multiple author books where you want a particular author to be the primary sort key.
That is one of the reasons the Edit Metadata dialog highlights the author-sort field in pink when the value on the books table is different to the value on the authors table. Just a hint that you should check that you want the book sorted by the value currently displayed.
Last edited by itimpi; 01-07-2011 at 02:03 PM.