Precisely how did 32 people manage to legally immigrate from the US into the US? Enquiring minds and all that...
My guess is that these are people who were born here, moved abroad and then renounced or otherwise lost their U.S. citizenship. Should such a person then move back to the USA, they would have to get an immigrant visa, and would thus show up in the stats as US-born.
Also a very few people (children of foreign diplomats) born here are not US citizens, and would thus need an immigrant visa (or lax US government) in order to live here.