For one they're definitely all "Sport". Now it's very hard to find period written material by BMW. In any case it's impossible to know with these cars which market they come from originally. I have found 2 official period files and it's Sport-Kabriolett in both. I believe they were just named by following up on what's already there and after searching modern info where today of course they're referred to as Cabriolet's. There's no real reasoning on how they're split...