I think (not totally sure, but I think) the dome light is always HOT and comes on when the door is opened, allowing the door jamb plunger to release and create the ground.
The electric path goes to the dome light first, then to the door jamb switch. If it went to the door jamb switch, then sent power to the dome light when the door was open, I think your wires would be reversed in the dome socket.
I might be way off though.
