I guess Columbia wanted to be specific. Since "South Asians" refer to people from India and its surrounding countries, and "Southeast Asians" to countries like Malaysia, Laos, Cambodia, etc, it wouldn't be accurate for them if they categorize themselves as "Asian-Pacific."
Asia-Pacific Islander is someone who has ancestry from West, Central, North, East, South, Southeast, and East Asia AND from the Pacific Islands.
All in all, I am also befuddled with the whole categorization, as these things changes from time to time.