TTT has it. If you're on a pair, then you have a circuit in that active equipment acting as a load, so there's less energy to radiate out into the air. If you split between two pairs, then you aren't completing a circuit, so you don't have that same loading effect. Depending on the circuitry inside the switch, you may see slightly different results depending on how you mix and match the pairs when you tone.
If you need to ID a cable plugged into a network switch, you can get testers that will blink the port on the switch. Or if you have two people, have one person blink the port by plugging/unplugging repeatedly while you look for the blinking port on the switch.
