I think it’s based in the way they’re partitioned. If there were no barrier between KGH and downtown Flushing (Mandarin Town) then I presume there probably wouldn’t be an eruv in KGH because it would be considered one town with well over 600k people. My understanding is that the LIE is essential in allowing there to be an eruv in KGH.
The boarders between Flatbush and surrounding neighborhoods have no physical barriers like highways; they’re just random, so they might all get counted as 1. As for the non-KGH Queens eruvim, I would imagine some of them are actually contested, but I admit I haven’t heard so either. It’s definitely always worth asking about before relying on one.