It's more reflective in how West Virginia classifies this highway: not as US 48, or a regular expressway, but as Corridor H, a road type by itself. The mileage is indicative of Corridor H's, not US 48. I'm not aware of other states doing this.
Correct. WV DOH (remember in WV "county route" is just a name for a road, still under state control, there are no county road departments, meaning a WV DOH county superintendent has a lot of responsibility.
So the DOH chain of command is, sort of, State, District, County. But the county only has responsibility for all the other roads. Parallel to the County is an "Expressway Orginization" with its own garage and equipment which operates as a sort of quasi-county.
The Expressway Orginizations will be named "Interstate (number) , Section (number)" or "Corridor (letter), Section (number)". With the borders ending at a specific mile post, not at a county line. This is the reason for the blue mile posts. In house, and to a great degree in the general public, Corridor roads are always "Corridor *" never their US route number.
As the state moves past the ARC allocation, US 35 now has a "US 35 Expressway Orginization" and, IIRC, WV 9 and US 340 (which have similar mile posts, but using the state or US shield in place of the state outline) share a "Eastern Panhandle Expressway Orginization".