Back in the old days (early 70's

) when experimenting with windspinners etc. one way of obtaining usable batteries with little cash for new or deep cycle batts was to use junkyard auto batteries discarded because they could no longer crank a car in winter.
The secret to rejuvenation was to empty out the old electrolyte, and rinse a few times with distilled water to remove accumulated sulphation sediment on the bottom of the plates, drain and refill with clean battery acid, or at least settled and drawn off clear. They then worked, not as good as new, but certainly as good usable batteries, in most cases. This can't cure plate damage, but it can bring the lower portion of the plate back into service where it is masked or shorted by accumulated conductive sediment, and that is a frequent ill of old or neglected batteries.
Charging stirs up old sediment and temporarily can clear the lower plate section -- if it will accept a charge at all. Sometimes to clear a some buildup the battery is charged for longer than its normal amount of time -- some marine inverter/chargers do this automatically for a specific interval -- I had a Heart Interface inverter that did this when I lived aboard.
I don't know why the battery in question increased its CCA after some discharge, but perhaps more sediment was stirred up by the discharge rate imposed after initial charging. One thing is certain, if there is sediment present, gravity works to re-deposit it on the bottom. The problems may return, particularly if the tractor sits for awhile and unpredictably, depending on how much "stirring there is, what the sediment looks like and how it arranges itself.
I have several chargers, and the newer "smart" charger is basically useless. It is stupider than the dumb ones and refuses to do its job unless conditions are perfect. Ahhh progress: smart everything, and made of plastic!