I've seen this happening for years and never paid any attention to it. But, after a user inquiry, I'm trying to understand what's happening.

When performing a global full text search for a name or email address, some items may not be returned/found. e.g. searching for Tom or tom@example.com returns many results, but a few messages are not included in the results. However, searching by From/Author for Tom or tom@example.com does return the previously missing messages.

The previously missing messages have Tom and tom@example.com in both the From field and the body of the messages in the form of headers from replies. Quick Finder has been run since the messages arrived.

Can anyone explain to me what's happening? Is this an odd quirk in the way that (global)find works or is there an indexing issue that I am unaware of?