That's not a bug, just not an obvious piece of functionality. Notice that it says "responses"? A pingback or trackback is a type of response. It's also considered to be a type of comment, just one left on some other blog.
To do this sort of thing correctly:
1. In your actual templates, where you call the "comments_template" function, change it to this:
The "true" part forces it to pre-separate the comments into types. This takes extra time, so you only want to do it when you know you're going to be separating them out anyway in the comments.php.
2. To display the comments number, do this sort of thing:
You can use any of those echo's to display the count for just that one type.