It might have something to do with landraces... If there is any genetic diversity within the population and the farmer is inadvertently using propagules from the best growing plants within the population. Or if there is inadvertent cross-pollination going on and the farmer doesn't realize because a year is a long time for a fickle memory to remember all the phenotypic traits of a variety, and then the farmer is selecting from an even more diverse population.
But lets leave those sorts of issues aside because people that grow garlic sometimes claim that it may take a few years to adjust to growing in a particular garden and that it gets better for a few years after arriving at a new garden. Since garlic is a clone and the DNA does not change, perhaps there is another explanation:
epigenetics, gene activation in the mother plant, due to environment, that is passed still-active through the embryo into the offspring. The underlying DNA remains unchanged, but how that DNA interacts with the environment may be modified by the experiences of the mother and/or father.
I haven't tried to measure epigenetic effects in my garden, but I sometimes wonder when I start localizing plants to my garden how much of the dramatic improvements I see are due to changes in the underlying DNA and how much is due to the activation of previously unused DNA. I definitely recommend doing the "Second Chance" grow-outs from seed that you have saved yourself. At least in my garden there are dramatic differences in environment between the typically damp/overcast growing conditions in Oregon -- where much Organic seed is produced -- and the super-arid brilliant sunlight that my plants are exposed to. I think it's very possible that the different environments might activate different sets of genes, and that those gene-activations can be passed on to the offspring.
Other improvements might be attributed to mechanics: The farmer learning what a plant needs and more fully meeting those needs. That certainly happened to me regarding fava beans and garbanzo beans. Both of them are cool weather crops that I planted for the first time during hot weather, after all danger of frost. Oops!!! They did much better when I planted them as soon as the snow melted. The farmer learned what the plants needs were and more fully met those needs and the plants thrived. The first year I planted favas I didn't get a harvest. The second year I got a meager harvest. This year the plants are already flowering, and it's still weeks earlier than
I even planted them that first year!