I can't find any reliable source to say why one is one and the other is the other. It's like the two terms are interchangeable.

Not sure if this is the right thread but 72sonnet's post ( http://imcdb.org/movie_115355-Silent-Witness.html ) reminded me that the car here ( http://imcdb.org/movie_239987-Silent-Witness.html ) should be on the other page.