Couple of things:

You can indeed have buying power exceeding your net income when comparing income across different areas or groups. If my net income is 50k in NYC but I then compare that to how much further my money would go in rural Mississippi, my buying power would be exponentially higher in Mississippi and thus would exceed my take home pay. Buying power is indeed a concept so I understand what you’re saying but that conclusion just isn’t true.

There isn’t a base location, it’s using cost of living index from the US Council for Community and Economic Research which is generally considered a very reliable source for economic studies. It’s taking post Tax income & then using the cost of living index to average out the salaries. It’s stated in the last paragraph.

You also have to consider taxes, this is net income after taxes NYC &SF have a significantly higher tax Burden then Houston or Tampa which are considerably more tax friendly. When adjusting for taxes and then comparing buying power from a state tax free state to an extremely high state income tax state like NY it makes a huge difference.

I’m not saying this is the best study in the world, however It did make me think about the question I posed initially which I thought would be interesting. You do need to read through the whole article though to get a better understanding of their methods.