Sometimes the field has subtle errors that make it treated as 2 different Authors.
The
Quality Check Plugin has tools to find and fix most of these type issues.
BTW one of the checks that you
should run is 'Check Author Sort'.
If you had changed the Sort rule, Calibre does NOT go and change older entries, only applies the rule to new content.
Other 'errors' are publisher caused
Same Author, but with /without initials, spacing or punctuation,
Then there is: Joe and Mary Smith. You need to fix those by hand