An under-recognized feature of honeybee inate instincts is their skill at adjusting the population to be proportional to cavity stores and size. (All tree hollows are not the same size.) They "want" to fill the cavity with functional comb and maintain a brood volume that keeps the population proportional to the supporting stores that can be stored in that volume. Tough job - but they make it work, if they don't have a beekeeper to contend with.
As a result of this skill, the bigger the wintering cavity, the more bees that are generated for seasonal activities. More bees equals more honey. They also have a safety margin in all critical survival requirements between what must be accomplished to survive, and the work force to get it done. In this case, the safety margin is in excess population. The surplus honey that you call your own is that honey created by the safety margin.
In TN, colonies can winter well in a deep and shallow. Further, if the broodnest is properly backfilled with nectar in the fall, the reserve of the shallow of honey is not needed in the spring buildup and remains capped through swarm season. It is a reserve,and can be used if needed, but remains intact in most seasons.
We might add that for either the single or double deep, properly prepared in the fall, Swarm prevention is quite difficult in the spring. In the single, all the swarm preparations take place in the deep and are hard to head off. In the double, swarm preps include brood in both, and prevention is complicated by broodnest disturbances of some kind. We switched to wintering in a deep and 2 overhead shallows years ago for the added flexibility.