Purchasing power can be misleading in countries with a top-heavy income distribution. Because cost of living calculations include everyday expenses but exclude most luxuries, average costs are close to what the typical consumer pays. But if the income distribution is skewed enough, average salaries can be much higher than what the typical consumer earns.
The US also differs wildly in cost of living on a state-by-state, city-by-city basis. Even the suburbs of big cities can have a substantially cheaper cost of living.
You're going to have a much different picture of this if you compare San Francisco and Montgomery, Alabama.
While you're right that median is more useful than mean in this case, it doesn't address the specific issue jltsiren raised, that this kind of statistics tends to exclude luxury goods.
This seems reasonable, as luxury is not related to the cost of 'living'. However doing so they include more-than-necessary-to-live earnings but excludes more-than-necessary-to-live costs.
In other words, if you include Bruce Wayne's income, you should include batmobiles, too!
