If you take a look at the subcategory report for the Appomattox County subcategory, you'll notice that the geodata is
(Inherited) from the county itself:
The most complete database of radio communications data
www.radioreference.com
That's also why you see
(County) on the subcategory details page:
The most complete database of radio communications data
www.radioreference.com
There are several places we can enter geodata. For conventional frequencies, the geodata exists at both the County level as well as the Subcategory level. If you don't explicitly enter geodata at the subcategory level, it will automatically inherit the higher level, which is the county.
It appears that inherited geodata does not make its way into the API, which is probably why it's coming up as 0.