Because the motors recieve both power and ground through the switches. So if you install relays, one relay needs to supply power when energized, the other supplied the path to ground, and then switch, hence pin 87 is power, 87a is ground. This works for the window motors. And also for the door locks.(if your lock relay dies, you can just build one out of 2 relays)
All you have to do is look at your diagram. Start by tracing the original. Power for drivers window up, leaves the switch on the blue wire, travels through the motor, leaves the motor on the brown wire and returns to the switch to ground. That's why the switches show the "up" "down" pins in contact with the ground. So when not energized, they are grounded.
So like wise, if your "up" relay is energized, your "down" relay will be off, and the brown wire needs connected to ground to complete the circuit, so like you said, in the off position, pins 30 and 87a are in contact.