X-Men First Class Meta: Erik's Accent

Watched XMFC again with my friend, G; I am now at the point of evaluating minutia, viz, Erik's floating accent. Whatever the production-level explanation for the migration of his accent from more American to more British and vice versa, the shifts in-universe are fascinating. In a nutshell, his English starts out very American, moves toward British/mid-Atlantic almost as soon as he meets Charles, then migrates back again toward American right near the end.

( light spoilers follow )