It took me a minute to figure out exactly how Sumner came up with this result. I assume that Sumner meant 2013 Q4 as the ending point of his second period starting at 2014 Q4, so that wasn't the problem. The problem is that Sumner calculated the NGDP growth at some point between 2011 Q4 and 2012 Q4 and at another point between 2012 Q4 and 2013 Q4, not the average growth during those two period (or any other meaningful number).