On a small city lot that I own, I found a pocket knife that was all ate the hell up at ~6". This thing was a goner. Granted, the soil out here isn't kind on most artifacts, but I've retrieved stuff from WWI that was in better shape.
Just about 10 feet away on the same lot, I recovered a clad penny at about 8 inches, and it looked almost new. What in the polka dotted eff is happening here? You don't know, I don't know...nobody knows. It happens. It could have been a recovery mistake (it wasn't...trust me), it could have been some fill dirt installed (no evidence of such), or it could have been...well, whatever. The mechanism that determines how things sink (or rise) is still not proven. Some argue that objects sink. Others argue that the ground rises. There's evidence to support both positions. I tend to judge finds based on how much the ground has influenced them (how badly stained they are by the iron in this area) and what was recovered near them at the same depth, but that's not telling the whole story either.
Figure out how this works in a way that everyone can understand and no one can disprove and you'll do us all a lot of favors.