I believe (and I'm willing to admit I may be wrong) that it has to do with voting trends and polling. The 15% that has been counted were probably enough cities and towns considered battlegrounds that once Sununu won those areas that the rest of the state would follow certain trends that statistically meant he won. AP doesn't crown the winner, the state does, they just felt comfortable enough in their projections with the reported votes to call it. It's no different than in MA, AP called Healy the winner as soon as the polls closed.