If there was a short through a thin medium it could add resistance between the wires.
It could also be that someone wired a switch in somewhere to put it in diagnostic mode and they did a shoddy job.
It could also be, since I believe one or the other of the A & B pins is a ground, that the wire is worn and grounding on the body but the connection isn't good enough to be a low enough resistance for the base diagnostic mode. I believe it's something like 500 ohms or below is the threshold.

0 resistance will cause the IAC to close fully. That is part of that diagnostic mode.