It's only confusing because BMW uses a weirder naming scheme than everyone else. According to their BMW USA website, they also call the 330i, 340i, etc as different models, but that wouldn't make much sense since every single car would be a different "model". I think it's just how they market them.
Take Mercedes for example. The S-Class and A-Class are distinctly different models. Then there's the S550, S600, S65 AMG, etc. They're different trim levels for the same car model. For BMW, the 3-Series and 7-Series are different models. But each of them have similar levels, 330i, 340i and 740i, 750i.