Why the High Resolution Total Vehicle Distance can be retrieved, but the standard Total Vehicle Distance cannot?

Why the High Resolution Total Vehicle Distance can be retrieved, but the standard Total Vehicle Distance cannot?

Clarification:
Both VD (Vehicle Distance) and VDHR (High Resolution Vehicle Distance ) represent vehicle distance. VDHR provides higher resolution, while VD is non-high-resolution.
For the gateway OBD service (for example, when retrieving OBD data via FlexAPI), distance is presented as a single value, and when the high-resolution PGN is available, VDHR is used by default and the non-high-resolution PGN is ignored.