The Rashbam on Bereishis 37:36 says that M'dan, Midyan, and Yishmael are all brothers from Avraham, though they do not share the same mother according to the Rashbam (Bereishis 25:1). The Rashbam also solves the difficulty of the numerous nations the psukim say bought/sold/transported Yosef by stating, on the next phrase in the Rashbam, that M'dan and Yishmael are one and are referred to interchangeably in the psukim. So according to him, we're really only dealing with two distinct groups: Midyan and M'dan/Yishmael.
My question is that it would have been simpler for the Rashbam to say that M'dan and Midyan are one (rather than M'dan and Yishmael), since M'dan and Midyan share the same father and mother (Keturah), while M'dan and Yishmael only share one parent (Avraham). Why, therefore, does the Rashbam say that Yishmael and M'dan are one, rather than saying that M'dan and Midyan are one (which would be equally effective at resolving the confusing psukim regarding the nationality of those who bought/sold/transported Yosef)?