My earliest childhood memory is of the swaying limbs of Golden Delicious apple trees. Rows of them stood next to a dirt road that separated our orchard from the front yard.
The Kentucky summer sky hung hot behind those high twigs, their leaves leathery green on top, soft as down underneath.
Frank Browning,“Sweet Temptation,” Reader’s Digest
Why, both down and underneath and not one of them; for example, underneath?
Is it emphasis? Sentence rhythm if you will?
"Down" has a second meaning here. It is used to reference down feathers, a type of feather known for being soft.
The phrase "soft as down" is just a poetic way to emphasize something being extremely smooth and soft feeling. The quoted text is using that phrase referring to the underside of the leaves.
The author is contrasting the tops of the trees with the undersides of the trees. The contrast extends to the materials he likens them to.
The leaves on top are like leather, pliable but not "soft."
But the leaves on the undersides of the trees are as soft as "down feathers," the small, soft, fluffy feathers that are under the contour feathers of birds.
" ... soft as down [feathers] underneath."
https://en.wikipedia.org/wiki/Down_feather
This is a false aperçu. The leaves at the bottom of the leaf ball are not different from those at the top -- not younger or thinner, and certainly not "fluffier" or "softer."
