There's a long-standing bug that got fixed before the recent change, but has come back with this latest change and has still not been fixed:
If you search for a term and ask for the results to show posts in which the term appears, and if the term appears in the wikipost, then every single post in that thread is returned regardless of whether the post contains the term. This appears to be because the appearance in the wikipost counts, and triggers a positive result for that post.
For example, try
this search which is for the term "skg" showing all posts on the BA board. The posts currently returned in the first 25 results (first page) are from these threads:-