Vehicle is parked on a hill in park the vehicle drifts backwards?

The transmission is properly locked but all it's holding is the input shaft to the transfer case. Sounds like they're using the better design viscous coupling between the front and rear drive shafts. Those have a lot of resistance to turning at different speeds so both the front and rear axles pull, but they're not locked solidly like the the transfer cases in trucks. You need the friction of the front tires on the ground, (and the front drive shaft), to keep the vehicle from leaving you.