I've noticed it before, but I don't know if its a performance feature. Perhaps in interviews Jon is trying to mask his accent.

Kinda related, but I had a teacher in high school from Newfoundland and he worked at masking his accent. There were a few phrases and words where it was blatently clear, and you would also hear it when he was tired. Other times, unless you were told, you'd never guess that this teacher was from the Rock.