It just means that they don't include buildings that are falling apart, nor do they include penthouse flats. Texas and Louisiana both have lots of dark purple and have the very worst public schools in the nation. It's really just a pretty good indication of where the jobs are.

Nearly by definition, someone has to go to very below average schools, and someone has to live in the very below average housing.

The first situation is more scary than the second, of course. Being young and single and in good health with no kids it was fine living below average.