Names in the tag browser (leftmost pane) are always sorted by the author's sort value. The displayed value can be the author's name or the author's sort value; this choice is determined by the "Use author sort in Tag Browser" (categories_use_field_for_author_name) tweak.
If the sort value is "Blogs, Joe" (common is Lastname, Firstname) and the author's name is "Joe Blogs" (Firstname Lastname order), and if you are displaying author names, then "Joe Blogs" will appear under "B". If you are displaying author sort values, then "Blogs, Joe" will appear under B.
If you haven't already done so, then reading the calibre FAQ entry
How does calibre manage author names and sorting might help.