Its not purely to do with UCI points, but also within the UCI areas, e.g Europe, America, Asia, etc:
“The top-5 American nations qualify:
The 5 first nations of the classification by nation of the UCI America Tour on 15 August 2013 excluding the nations qualifying via the UCI WorldTour classification: the 1st and 2nd nations qualifying can enter 9 riders, with 6 to start (rank 1); the 3rd, 4th and 5th nations can enter 5 riders, with 3 to start (rank 2).”
Colombia has already qualified via the WorldTour, so is omitted from the America Tour nations ranking for this purpose. That gives the United States and Venezuela 6 riders, while Brazil, Mexico and Canada each get 3 spots. ”