I think the way it goes, is that you're born a Gallifreyan (or loomed as far as that idea goes), but that you don't get to be a Time Lord or Time Lady until you get through the Academy, which takes a good bit of decades (there are outsider groups on Gallifrey, including mention of a kind of "hooligan" type class called Shobogans, who I don't think are considered Time Lords but who are Gallifreyan in physiology). So Susan, without her full schooling, maybe was considered a Time Lady in training? There's also the complicating factor that there is canon evidence to suggest the Doctor is half-human to begin with, so his family might just be weird as compared to the typical Gallifreyan lineage.

/ubernerd