The model seems to be more recurrent on the 987 when you Google Image search "Boxster 987" which leads to believe that they are Boxster's factory wheels.
Then who knows, might be one of those 'option' offered by Porsche at the time. For instance I think there are more Boxster 986 with Carrera 18 wheels than there are Carrera cars out there.
Not that it matters mate, they are nice, you like them and they work! All what matters really
Congrats