That's because of his position. He doesn't suddenly "find" speed - he just holds back before because he doesn't want to risk an overtake. Button is the only one - save, at times, Barrichello and Vettel - who consistently had the better car. If you watch previous seasons, you'll see Kimi, Alonso, Massa, Hamilton and the rest have all done it when they could and needed to. It's a common strategy when you have a faster car - fuel a lap or two longer, cling behind the guy, and once he pits, unleash the full pace of your car while he drives around on a filled tank.
Only the faster car of the two can complete such strategies: The slower car will drive at the limit anyway. Jenson's strategy isn't new, it's just new for him. Nothing to write home about, anyway.