Why does it seem that most of the coins are around 5-6" deep. I went out to the football field on a naval base Saturday and Monday. Most of my finds were 5-6" deep came out with around $3.60 in clad, a Masonic pendant, and a Catholic pendant.
Years ago, on this forum, it was said that for every hundred years of time, about four inches of soil is added to the Earth. Obviously, soil erosion from wind and water are a factor. Now, figure the age of your coin finds and it could very well be in line with that estimate.
Type of soil I think has a lot to do with it.......clay--sandy--pumice----then you have the weather facter and shrubbery growth. Like the high desert of Oregon here, don't have too fight the brush much, not that much of it......
Coin depths can be anything really, I've found 100 year old coins on the surface and I once dug 6 inches in my own front yard only to find a 1994 dime(I was pissed)....My yard is really wet with soft dark earth...you can push your Lesche in to the hilt with one finger...other places near me you need to pound them in with a hammer basically.
You should rework the area in all metal and see whats deeper. Gold rings and coins will sink faster than the finds you have been making though as Coinstriking says depth can have nothing to do with how long an item has been in the soil. There's wind effects, vegetation, floods, frost cracking of the ground even a cow treading on a modern coin in wet weather which will push it down many inches. All proves that the archaeological worries re metal detecting have no basis in fact.