Eruvin Made Easy part 5: Flatbush vs. Kew Gardens Hills

According to Rav Moshe Feinstein, an area is assumed to have 600,000 people if there are 3 million inhabitants in approximately 12 square מיל. Why did Rav Moshe rule that Flatbush is viewed as part of Brooklyn and is not eruv-able whereas Kew Gardens Hills is not viewed as part of Queens for eruvin?