I don't necessarily have a problem believing that you're getting traffic from forum signatures, although I, too, would like to see more than just your unsubstantiated word for it.
But what makes the difference is: what does this forum signature traffic do
after they land on your pages? Does your forum signature traffic convert well?
It's easy for somebody to click on a link they see in a forum signature -- for instance, when you read someone expressing an opinion, you might want to visit their site so you can get a better feel for their overall credibility and credentials before you decide to accept or ignore their post. But for someone who wants to generate conversions, that sort of traffic isn't exactly useful.
So good for you if you're getting traffic from forum signatures! But much more impressive if it's actually desirable