Area in the name refers to the first definition I found on google define: area
" a region or part of a town, a country, or the world.
"rural areas of New Jersey"
synonyms: district, region, zone, sector, quarter, precinct;"
not the second
"the extent or measurement of a surface or piece of land.
"the area of a triangle"
synonyms: expanse, extent, size, scope, compass; More
What you what is the cache density per area (cacher/km^2)
That it not available on pgc. There exist to place caches in an area but I guess that calculating area from that will result in some problems.
One is water.
If you look at the borders on the Map Counties tool you will see that the size of the polygons at the coast. How much water in included varies. The result will be quite different for inland and coast areas.