That is the average depths that a coin is found. That means that most people cant get below 6 inches. Many detectors are not very strong, or just cant get very deep, like many VLFs. Most PIs will go very deep. But it could be the user. I see many people swinging the coil like a sickle, in a wide U shape. At the closest, they are 6 inches from the ground, at the end they are over a foot off the ground. Sometimes it is the user, not the machine. Or they just need to listen closer to the thresh hold going up slightly, or go slower, or really overlap the swings. There are some deeper obviously. So use a larger coil /or a more powerful detector and go deeper.