Battery terminal corrosion is most often caused by contact with the corrosive (and explosive!) gases and vapours which vent out throught the openings in the caps during the normal charge/discharge cycles. It also can happen when the acid is splashed up on top of the battery by careless handling or vehicle movement, although most caps have built-in baffles to minimize this. A semi-conductive layer of dirt on top of the battery can aggravate this situation by providing a pathway for current flow and galvanic corrosion.
Note that not just the terminals, but anything else in the immediate vicinity of a battery under the hood of a car that can corrode, will.
Also, the life span of a battery which is exposed to high under-hood temperatures is greatly reduced, so a well-placed battery (such as under the seat) in a tractor should easily outlast a car battery under the same usage conditions. (Also puts the weight more over the rear wheels for better traction.)
Motorcycle batteries, and some car batteries (such as my car, which has the battery in the trunk) have a separate fitting which takes a vent tube that releases the gases either away from other components or to exterior of the vehicle. This keeps these batteries in as-new appearance for years right up to the day they expire. On a motorcycle the tube also prevents acid from spilling around on electrical components when/if the bike is tipped over.
Like yours, my Kubota battery terminals remain spotless, with very little effort on my part, but I expect that, unlike a car battery which is contained under the hood, it is because it sits out in the open and the gases can freely move away from the battery and other components.
Are the other JD tractors that have a corrosion problem set up so that the gases are contained somehow, such as they would be under the hood of a car? It would be a simple matter of replacing it with a compatible battery type with the vent tube fitting and installing a plastic tube to carry the vapours away. (Preferrably the next time the battery needs to be replaced anyway.) Although, these batteries do cost a little more.
Hope this helps.
Regards - Brian.
