Subversion has an API that is called here to determine what revisions are eligible to be merged. From the command line, it would be something like:
$ svn mergeinfo --show-revs=eligible ^/branches/branchname .
The equivalent API for this command is used by the merge client to populate this display. So yes the answer in the comments by @LazyBadger covers it. The fact that you did not see the revision is a sign that it had already been merged (or the revision touched a different branch then you thought).