Sounds as if you have some wires making contact with each other. Lets say, zone 5 is on and you are seeing voltage on zones 4,5,6 and 7, then the implication is that somewhere the bare wires are in contact with each other so that the voltage can jump from 5 to 4,6 and 7. I have seen this happen if someone strips off too much insulation on the wire at the controller and the bare wires end up touching, or in the valve box. I'd check those spots first. If you spliced any wires when you did the install, you may need to dig them up and check the splices.