Sylar got to finish the cut with Claire. In Peter's case, he stopped and then was distracted by Mohinder.
Or maybe, in five years, Sylar has improved his cutting technique. He said he was waiting for Claire. He must have figured out a way to do it.
We do know that the wound won't heal as long as there's something still breaking the skin. So maybe he used that. We can't be sure because we didn't see what happened after he made the first cut.
