I think that Obama when studying in Hawaii might have gotten the figure 57 from being taught about all the parts of the United States that vote or are territories, including Washington, DC, Puerto Rico, Guam, American Samoa, US Virgin Islands, Northern Mariana Islands. This comes to 56. Maybe there was another when he was in school or he forgot the right number. Certainly there would have been more emphasis in school placed on these Pacific Island US territories as being worth remembering.
That's unlikely, since Hawaii is number 50. It also smacks of desperate rationalization.
Panama Canal Zone, until Carter gave it away.................