Could be wrong on this but one possible explanation is based on the understanding that
one of the Thurles-Clonmel bus routes is de facto a rail replacement service.
Perhaps the reason for the non-availability of a fare based on the closed route distance (43.85kms./27 1/4 miles) is to prevent abstraction of business from the bus service.
There may be some such condition dating back to the rail closure, which incidentally this coming September will be 50 years ago.