When I saw her live a few years ago, she explained the story of it. Basically, she was at summer camp and one of the officials was a young Brit from Liverpool. UK students used to do that in those days during their summer break and, well you can guess the rest...
Also a big Green Day fan. Loved the comment when they first started out about how they couldn't be a real punk band because their front man was too good a singer :-) True!