Well, what doesn't make sense is, Luke's little nod of relief upon seeing his father appear in spirit form... Luke doesn't know what young Anakin looks like. To him, his father looks like that old man he JUST burned up moments before. Now, Luke is the ONLY one who can see these spirits. Right? Leah didn't see the spirits. (Leah actually never even MET any of these people. [well, technically, Vader, but he playing the part of her father.)] So, the form the spirit obviously takes would be one that the intended viewer (Luke) would recognize, thus, in Luke's case, the older versions of these characters.