I was just down at the LHBS getting supplies for the next batch (Ipswich Ale clone), and talked to the guy that runs the place about this exact issue. (It was a recipe of his that I used) His first question was "How was the carbonation?"
My reply of "Not bad, but I'm sure it'll get more bubbles." Triggered his response of "It's probably still some of the priming sugar. As more carbonation is formed, carbonic acid is formed, which will also help moderate some of the sweetness."
Makes sense to me.