Overtime in NBA games is a frequent occurrence. A bit too frequent, as it turns out.
But overtime is almost 3 times more frequent than this: 6.26% of all NBA games are tied at the end of regulation play. And games decided by just a few points are surprisingly rare: It is more likely to have a tie than for the game to be decided by two points, and a tie is more than twice as likely as a one-point difference. These statistics are quite dramatic when you see them visually.
The reason for this is strategy. Read all about it here.