Here is my opinion.
For one as stated by others above the make-up/content of the soil is a huge factor.
Then how much rain/moisture is a factor.
Whether or not your on a flat area, hillside, hilltop, low lying area etc. A hill-top or hill-side will not hold the water like a low lying area so hence with the added moisture help things sink faster.
Then factor in the density of the soil as some is hard compact some is loamy.
You also have the freezing and thawing if your in that kind of climate.
You also have to factor in hole much rock is in the ground how many roots are in the ground etc. etc.
Now, some probably will not agree with this but on buttons, and smaller coins I generally find them deeper then say LC's or Halfs etc.
The reason is I believe the density of the object has alot to do with how far it can sink.
My oldest coin is a Realle and it was only about 6" but, it was on a hillside. My deepest is a Half Dime and it was just about 10" in a low lying area surrounded by other areas sitting higher up holding less moisture where LC's where being dug at 7-8" or so.
I will say when this coin was dug it was on my third day if I remember correctly after pounding the tar out of this productive spot. It had rained the day before and the conditions where great. I am suprised I found it but, I do think it was the soil moisture content that helped. It was a repeatable one way signal that bounced mid high to right mid on the SC. It was not loud but faint but good sounding high.
I am a firm believer in the conductivity idea that coins do produce a halo effect and the more moisture surrounding that given coin/object the better it can be detected.
I will also add I have pulled LC's at about a inch in very very hard compact clay soil.
In general though I would say 10" is not the norm for me as most are about 6-9".
This is all just my opinion so, take it for just that.
But, do not beat yourself up over 8" old coins that is fantastic.
I could care less how deep I dig but, rather just if I am getting the old coins.