"The Cost Conundrum"

Why are health care costs in McAllen, Texas twice as much as costs in El Paso County even though they have similar demographics?:

The Cost Conundrum, by Atul Gawande, The New Yorker: ...McAllen, Texas ... is one of the most expensive health-care markets in the country. ... El Paso County, eight hundred miles up the border, has essentially the same demographics. ... Yet in 2006 Medicare expenditures (our best approximation of over-all spending patterns) in El Paso were ... half as much as in McAllen. ...