There's another angle that could have an effect on the depth of coins. Living in New England, we are constantly dealing with a natural movement of the soil in the winter that causes "frost heaves". As I understand this, the hard frost in the winter can make the soil and rocks (and buried coins, I guess) push up toward the surface and that causes the pavement in the winter to become very bumpy in spots. Now, it seems to me that if all this dirt and soil is being pressed upward, then deeply buried coins and other parapenalia would also be pushed upward. This is just my theory and, sometimes I admit, my wife gives me way too much time to think.
Steve