As I wrote in the other topic, the Hellenic elites are not well documented: we have to make due with occasional and sometimes contradictory references rather than full descriptions. My guess is the team decided to create three types of elite units the successors can be expected to have had (an elite phalanx, an elite heavy infantry and an elite mobile infantry unit), and gave these the names that seemed to fit best (argyraspides, the old-fashioned hypaspist and the more modern peltast respectively).