The default setting, wildmode=full , highlights (if anything) what it actually completes. If you repeatedly hit the right-arrow key, it will cycle through all full matches and what you had typed. See ":help 'wildmode'" for the various other possibilities. Whatever you see highlighted on the statusline, what is filled-in is what you see on the command-line. If "longest" is included, it's not necessarily a full match. The purpose of that is to allow incremental completion.
Yes, I understand the purpose and usage of wildmode. My suggestion is that the wildmode 'longest' should behave like the completeopt 'longest' where an entry is only highlight if that highlighted text is actually inserted into the buffer, or command line in this case. As of now , the behavior of 'longest' is inconsistent across the two vim options ('completeopt' and 'wildmenu'). -- eric