Hopefully the 3 phase feeds for each controller come off of the same switch gear. Each controller will use a main transformer supplied buy its individual 3 phase disconnect. This controller transformer will typically take two legs of the 3 phase and convert that voltage to 120VAC for controller voltage. If one elevator is utilizing say, phase #1 and phase #2 of the 3 phase power supply, and the next elevator is utilizing phase #2 and phase #3 for its power supply, you now have a difference of potential between these two controllers. When these two controllers are connected via group connections, you have a doubling of voltage between the shared circuits.