Is this a bug? How is the percentage calculated? Since C++ is one of the most important skill in my bag, this is pretty relevant for me.
You've listed c++ under the "dislikes" section of your CV, which causes it to be removed from your Top Tags list.
There are some additional critera that determines whether or not a tag is shown, but both you and the c++ tag itself should easily meet those. While the fact that you have c++ in your "likes" as well should perhaps void the dislike filter, the easiest fix is to just remove it from the "dislikes" section.
To avoid confusion on this matter, and to give the user more control, it might be better if there was an indication that this was the default behaviour and if there was an option to disable it, like in the following mock-up: