Just a thought, All of these lights will almost certainly be run from the same AC socket. If not then definately in the same building. Each one will have its own transformer though. Is that what causes the ground difference?

The total distance between the master and the final slave wont be more than 50 meters.

Im not quite sure i understand ground difference properly. Each device has a transformer so its isolated from the mains. i cant see how i could get a reading accross 2 grounds unless im thinking of something different. Im sure this is explained in the rest of that document though