The answer is obvious.
Take for instance Ramot. Until a few years ago Ramot kept regular Purim. However, once there were enough developments between Yerushalayim and Ramot, ramot fell into the Yerushalayim jurisdiction and now keeps Shushan Purim.
So to, as the borders of Flatbush ir haKodesh expanded, they absorbed other nearby boroughs such as Midwood and Kensington.
This also explains why Midwood and Kensington also keep the same day of Purim as Flatbush.