Don't take me 100% on this. It's really by choice. But the Spider edition of a Ferrari is a choice it isn't the actual car. For Maserati however, the Spyder is the actual name of the car, it isn't a type of the car.
There is a discussion about Spider vs. Spyder here at http://hem.passagen.se/veloce/WELCOME.HTM supposed to have to do with politics.