I discovered this in a glossary in one of our testing sites.
This glossay has an entry whose concept is "Analysis". The thing is that the actual concept is "\ufeffAnalysis", it has an "invisible" unicode character at the beginning. In Moodle web, this entry is displayed in the "Special" category when sorting by letter.
However, the WebService mod_glossary_get_entries_by_letter returns this entry along with the entries that begin with "A" (at least when retrieving letter=ALL).
IMO in this case it would be better to "trim" the concept and always display it in the "A" letter (both in web and in LMS), but I'm not sure if that's easy. In any case, it should have the same behavious in both cases.