Might also be down to allies and who was in the elites. The romans were friendly with quite a few tribes and allowed them to stay in charge etc. They were romanised, but maybe linguistically they held on to celtic as well so the elite was mixed. Ditto normans with some saxon nobility and clergy. (Although probably less.) I don't remember reading about saxons doing something comparable, but then I don't really know much about the saxons. Just an idea I thought I'd throw in