I honestly don't know the reason(s?) for planting at certain depths. All I know is, is that it does make a difference.
Keep your planting depth just below any grafts...although I have a pear tree that shows 4" or so of trunk below the graft and it's doing well.
Otherwise, if you're just moving a plant, keep it at about the same depth as it was before.
As the ground subsides or shifts or the plant "moves up" due to root growth, I just add more mulch. Need to do that with the pear, but am waiting on the rest of the leaves to fall.
Most fruit trees like a little slope to the soil to keep water from standing around their roots (peaches are the least tolerant of all), but anything over 8-10" of soil over clay is acceptable. Obviously, the deeper the soil, the better...