15 seeds are 7-117 against 2 seeds. That's a winning percentage of 5.6% - so about 1 in every 18 games. But with the addition of two sixteen seeds, you essentially have two 14-seeds listed at 15. And likewise with 16 seeds; there are essentially two number 15 seeds listed as 16-seeds.
There have been several close games over the years. Gonzaga (2013), Syracuse (2012), and Pittsburgh (2009) all escaped with narrow victories recently. In 1989, Oklahoma and Georgetown defeated East Tennessee State and Princeton, respectively, by one point each. On top of that, Illinois only beat McNeese State by six points. Three Number 1 seeds won by a combined 8-points in the 89 tourney - crazy. Perhaps this is the year where we get one or two nail-biters in the 1/16 matchup.