Archmage /was/ an 18th level magic-user, back in the day. Of course, Wizard was an 11th level one. And 'wizard' more than 'archmage' conjures(npi) images of a wise, magically capable elder like Merlin more than a precocious young apprentice (Harry Potter notwithstanding) so it doesn't really make sense for a 1st level character, either.
The point, though is still valid, in that a character class name is something that shouldn't necessarily be (or even necessarily shouldn't be) used in character or be linked to the fiction of the world, but, IMHO a PrC name should be both.
A Purple Dragon Knight should ride around Cormyr, wearing purple, bedecked with heraldic dragons-rampant, and introducing himself as - or better, having his herald preceding him, loudly proclaiming that he is - "Sir Wassisnaym of Cormyr, Purple Dragon Knight!"
...cause, y'know, it's prestigious.
In contrast, a 'bannerette' could be any third-level fighter with a dram of leadership talent, or maybe a kid twirling a flag in a marching band*.
*arguably also 'prestigious' if you're marching in the Tournament of Roses Parade or something, just not in the same sense.