The Explorer especially on ID doesn't like disturbed ground. I have coin plots that have been there for several years and my Explorer doesn't necessarily give a textbook hit. Not only the Explorer but my single freq detectors have a hard time correctly iding some of the coins.
My theory is that if a hole is dug, then a metalic object is placed down then covered, it will disturb the natural ground matrix for a long time. Now how long, I don't know but I remember reading where they can take GPR and tell what areas of a historic site have been used, going back hundreds, perhaps thousands of years and which areas the ancients didn't disturb or use.
My point is burying items, even if it has been years is not the same as naturally buried items.
I have tried to duplicate tests where nails and coins are buried together, burying them then paying attention to the results.
What I found out the hard way is even though burying objects can give a general idea about a detector, the true test is in real life situations.
I think test plots are fine but don't put too much emphasis on the deeper buried objects especially with meter readings and such.
One other note is on the Explorer, the half dollar and silver dollar has a lower conductive reading on them than a quarter, do an air test and you will see.
The Explorer doesn't have the traditional meter system where as the coins get bigger, that they are neccesarily higher in conductive value.
There is a chart around here somewhere on the Explorer ID, someone help me out.........