In the electronics world very high impedance is a wonderful thing. You meter does not affect what the circuit is doing. In "power" circuits, this high impedance is not so great. You can see induced voltages on "floating" wires that have no effect. I am very surprised digital meters meant for the "electrical" trades don't have a built in load resistor (maybe switchable) that makes these "phantom voltage" readings go away. A megohm would probably do it and still have negligible affect on your readings.
