It's nulling on the nearby iron nail (which is discriminated out or low tone) depending on sweep direction, recovery speed, separation and relative depth of the two targets. Calabash's test garden has both lone non-ferrous targets and also ferrous/non-ferrous targets in the same hole. If there is little separation between the nail and target and they are both under the center of the coil, the iron will tend to dominate the signal causing a null or low tone. Since the nail is long and thin and the coin is round the response will be different depending on which direction the coil approaches the target. That is why a 90 degree turn on an iffy target signal next to iron will sometimes clear the signal up.
Lone iron targets also sometimes present false high tones in one sweep direction off their edges either due to the flattened nail head end or if the nail is bent. So one way signals typically indicate the presence of iron with the possibility of being co-located with a non-ferrous target.
Finally, really deep non-ferrous targets depending on their orientation in the ground (like an on-edge coin) may only sound off in one direction.