"And he's a British butler for Odin's sake! He's been trained since boyhood…"
Avengers #54 (original series) explained that Edwin Jarvis was actually a poor kid from Brooklyn. It's subsequently been stated that he left home to join the Canadian Royal Air Force and ended up in Britain. Presumably the idea there was to rationalize why some writers erroneously give him a "British" dialect…but the real reason is that lazy-ass writers who can't be bothered keep confusing him with Alfred!