The data is purporting to show total property value, not land value. Thus, a residential tower with 400 units at $100,000 each and occupying five acres adds $40 million to the valuation. In contrast, a five-acre spread with a single mansion might only be worth $5 million. Thus, a county with more residential towers will have a higher aggregate value than a county with more luxury mansions, even though the former is much more affordable.
The map shows total land price in the county. It doesn't measure anything like affordability of land (which would be measured by something like price / unit area of land) or residential affordability (which would be measured by something like price / unit area of home.)
