They survey people.
Their own website claims that they
talk to over a million people ever year.
It seems they start by asking their magazine subscribers but periodically go outside of that to talk to non-subscribers too. I see absolutely zero reason to suspect that they do all of that within a single city.
If you have some reason to think that, but all means, explain what it is. But at the moment it looks like you're just making up stories that have no basis in fact. Do you really think that ALL their magazines are sold to just one city?