I have a F70 & I planted a quarter down 6" & the ID went just about everywhere except iron! Would never have expected it to be a coin. The deepest coins I've dug are 5 1/4" for penny & 5" for dime, but I don't dig the ones that bounce down to pulltab or below. Guess maybe I should when they're deep.
My ground is horrible for consistent IDs unless the coins are very shallow. My F70 can only tell the difference between a nickel & a modern tab if they are on top of the ground. Most of the coins I get down 4"-5" will bounce from one denomination's range to another. Whenever it jumps between ID ranges the confidence bar goes down to 1 but these have mostly been coins.
Also, my F70 often claims the coins are deeper than they are. I'm guessing the minerals or whatever in the ground make the coin signals weaker so the detector "thinks" they're deeper, based on signal strength.
I'm not knocking the F70, as nearly every detector I've owned has this bounce in my difficult soil, to a greater or lesser degree. HH, George (MN)