"A small number are handled under the 505(b)(2) statute which does allow for generic drugs"
I don't think this is correct. Such a drug would be "similar", but to file under 505(b)(2) the applicant company would have to submit not an aNDA, but a full NDA or BLA, meaning that he does independent clinical trials with the product, rather than depend on the trial data of the copied drug.
Such a drug is usually developed as an improved re-formulation, at quite a degree of expense (unlike the usual generics) and time, and often the intent is increased bio-availability or reduced dosing convenience.