I've noticed the same thing on some sites. To prevent it, add a terminal / to the URL, like this:
That prevents the behavior you noticed on IIS. What I don't know is why this happens. Something in the IIS <-> ASP.NET interaction tries to encode the dash in a non-slash-terminated URL, but not if there is a terminating slash. It may have something to do with IIS not having to redirect a slash-terminated URL.