It is my understanding that Pediatrics is general pediatrics (from which you can be a hospitalist, a primary care doc, or go onto fellowship) and Pediatric-Primary is a pediatric residency track that focuses specifically on primary care (so it is probably difficult to go on to become a hospitalist or enter a fellowship, with more extensive training for the primary care setting).
One is more competitive because there are few positions available, 65 vs 2428.
As an aside, this is a perfect example of how difficult it can be to interpret this aggregate data. That list makes it appear as though Pediatrics Primary [care] is the most competitive residency in the US. To some extent that is true, but in actuality you can practice pediatric primary care straight out of a general peds residency too, which isnt terribly competitive. Throughout your medical training, be a skeptical consumer.
-senior medical student (t-41days)