mbachur:
This is the same like the din that if you go live in E”Y but go to America for Yom Tov withthe intention of returning back to E”Y then you only keep one day.
Similarly, most of Florida especially Miami is made up of snowbirds and so most of the city would be keeping the same day of Purim as Flatbush. It could be the minhag spread to the people that live in Miami to also keep that day since they are a miyut and decided to take on themselves like the rov because of ‘al tifrosh min hatzibur’.