Pete, I don't know the precise answer to your question......but I will take a shot at it.......we road bike riders when we have an off "tend" not to face plant and are on a surface that is hard and will allow us to slide to a certain extent.
Adventure bike riders are still travelling at a substantial speed on softer, rocky, uneven surface that generally won't allow you to slide and wash off inertia, they also tend to face plant more so than a road rider.
The other reason now presents......because they tent to face plant more often by increasing the chin bar distance from the riders lower jaw area......the helmet is less likely to travel rear wards from inertia causing lower mandible, maxilla and face trauma.....the chin piece on these is a sacrificial structure designed to absorb impact and not transfer it to the wearer.......much the same as a modern car in a collision, front of car is designed to absorb and protect the passenger shell.
Does that help ??