As someone who tries to keep the vague number in mind, it would be strange to me as well, but I suspect a large number of people don't really try to keep even the vague numbers in mind about how many people are about or how many people realistically could reside in a place like NYC.

They track the rough oversimplifications. Like "barely anyone lives in the middle of the country", and every TV show they see in the US either has a bunch of background people in NYC or LA, or is in the middle of nowhere with a town seemingly made up of mere dozens of people. They might know that "millions" live in the US and also, "millions" live in NYC, so same "ballpark" if they aren't keeping track of the specifics. They'd probably believe 10 million in NYC and 50 million nationwide.

This is presuming they bother to follow through on the specific math rather than merely roughly throwing out a percentage.