What causes some nails to high tone?

You wave it across the coil and the detector is 100% sure it's iron, then drop it back in the hole and it's a sweet high tone?

Will your detector get a hit over the empty hole. My Gold Kruzer will hit on an empty hole that's been dug. Even after it's filled it hits the ground. My ground is mild but it must be a change in mineral in the ground doing it. I don't know if that's what's happening with the nail.

That's the $64,000 dollar question?

Orientation of the nail.
Ground Balance.
Ground Balance offset.
Mineralization.

Magnetic response still very strong.
Typically nails 'disintegrate' to a wet mush and are non-magnetic / non-responsive.
When a nail is made with 'extra' heavy content make up, disintegration can take many years longer and are very magnetic = strong signals
We used get a lot of complaints from the 'Italian's' and the 'Russian's' because 'hob nail boot tacks' (RU) had the same 'signal signature' as tiny inscribed hammies they used 'store' in their mouths!