When you perform a search with an excluded tag, the tag information box appears.
For example, do a search on this meta site for "unicorns -[discussion]". At the top of the results page, the information box states:
This tag indicates that the question may not necessarily have a clear-cut right or wrong answer and is often subjective. If it's not a bug or feature-request, it is probably a discussion.
Describing the tag that was explicitly excluded seems to be a bug. I noticed this when searching on StackOverflow for "layout -[HTML]". When I saw the HTML tag information box at the top of the page, I mistakenly assumed that I had used the wrong tag exclusion syntax.
Note that this does not happen when using two tags: "[layout] -[HTML]" has no info box.