Too Much Continuity

Here's a question I have about Heroes. If you've not been watching it lately, and you don't want to know, obviously stop reading.

But I'm puzzled by Adam's accent. When he was first introduced, as Kensei, I assumed he was a time traveller: his accent and dialect were decidedly modern-day English, which I assumed didn't exist in the seventeenth century. (I don't really know how I would check this.) Eventually I just assumed he wasn't.

Then he lived in Japan and America for four centuries, and hasn't picked up an accent at all? I for one think that's weird. Does his accent have healing powers of its own?

It would have been a nice touch if his accent had changed a bit between the 1650 and 2008. And the actor is American, so I assume he can do an American accent.