Probably has to do with the problems the N54 had with the fuel pump and wastegates on the turbos which were a big problem on the 335i. It seems the problem with the fuel pump had been somewhat corrected and the turbos/wastegates got an extension of their warranties.
However, this data is based on incidence of problems and it is compiled the same way across all car models. As long as they have adequate data, it should be broadly representative. Of course, a single sample could be better or worse. I think the issue that is being highlighted here is that BMW spends most of its time on engineering high quality in terms of performance in engine, ride and handling. It would seem less time is spend on reliability.