The info_lighting FGD (and the lighting origin key value) describe lighting origin overrides as follows:
An entity that can be used to change the lighting origin of a prop_static. Set the prop_static’s Lighting Origin to point at this entity to make the prop_static light as if it was at the info_lighting’s origin. Good for prop_static entities that are embedded in world geometry (like rocks/windows/etc).
This, however, does not seem to be what the entity actually does, as the amount of light on the entity varies with its distance from the lighting_origin.
Here, I have a lighting_origin and a light in a completely self-contained box outside of the main map, and a bulletin board with its lighting origin keyvalue set to that entity- if the lighting origin system worked as advertised, that should mean the bulletin board would always be the same brightness (since the lighting at the info_light ent is completely determined by the single light in the box, which is kept constant). However, when I move the physical location of the box, light, and info_lighting while keeping them all fixed relative to each other, the brightness of the billboard changes.
Here it is with the box at the edge of the map:
Is this intended behavior for these entities? If so, why?