The Porsche theme overrides the original theme on another device because it is made to be the original theme on the Porsche; it is a ported theme, not a created theme from Theme Studio (or other theme making software).
I actually went to the Porsche Design store in Beverly Hills today and showed the guy the theme. He tried to tell me that my phone wasn't as powerful as the P'9981... he wouldn't accept that they had the same processor/memory and that the P'9981 just has a different look.