bibtex-sort-ignore-string-entries

bibtex-sort-ignore-string-entries is a variable defined in `bibtex.el'.
Its value is

t

Documentation:
If non-nil, BibTeX @String entries are not sort-significant.
That means they are ignored when determining ordering of the buffer
(e.g., sorting, locating alphabetical position for new entries, etc.).

You can customize this variable.