New Horizons wasn't actually designed to collect so much data. Since its launch new moons have been discovered, and obviously that's a bit of extra science. Plus, as they said yesterday, they have discovered dozens of ways their instruments could collect even more data (for example after the Jupiter flyby they discovered Alice could also be used as a detector of some kind of particle I forget). Shouldn't make a huge difference, but still, it's more than they expected back in 2006 and when they were building it.