i agree with birger; the arms of rulers weren't necessarily the same as those of the realm. think france: she goes from a semy of fleurs de lis to only three (according to one tradition in honor on the holy trinity, but anyway), but her rulers have many other variations.
in most cases the monarch's arms were to show his/her pedigree, or that "state {a} is married to/allied with/vassal of/suzerain of state {b}"; this can often change from one monarch to the next, but the state, barring any diplomatic annexation, concession, etc., remains static.
i'd not worry about each successive monarch and focus on events like the hundred years' war, for example, that caused a change in england's arms as she felt france to be as integral to her as say, wessex.
steph
