JBD is correct on selective breeding. Tobacco, like all plants and animals, also uses non-gene materials (epi-genetic) to regulate not only the expression of the genes that are present [a particular genetic trait may be present in the genome, but not allowed to develop], but also the subset of genes that end up in its eggs and pollen. This epi-genetic impact on future generations is subtle, but does occur. Epi-genetic effects are often regulated by growing conditions or herbivore predator impact.
Although heirloom varieties have a fairly predictable set of genes from one generation to the next, a good way to think of its genome is like an omelet, always made with the same quantities of the same ingredients. When you cut it in half (I eat one half, you eat the other), which is analogous to the process of creating eggs (no relationship!) and pollen, you may get more cheese, while I may end up with extra mushrooms.
Even with careful bagging and knowledgeable plant selection for seed, and from a large number of specimens, every variety of tobacco grown by an individual grower will gradually "drift" from the original specimen.
I've grown PA Red using seed from two different sources: 1) New Hope Seed and 2) Nicotiana Project. The plants from batch 1 were all consistently taller, with wider leaves, while the plants from batch 2 were all consistently shorter, with longer and somewhat darker leaves. They are both easily recognizable as PA Red, and taste pretty much the same. But the two genetic lines have drifted apart in subtle ways.
Seed from a single seed pod is not genetically the same, even when "selfed." Given the huge number of seeds in that single pod, the entire lot of them likely has representatives of every possible combination of traits available within that individual plant's genome. When you germinate, say, 50 seedlings from the lot, every plant will be genetically unique, though most of them will strongly resemble one another, as well as the parent. From these, you can easily select a "typical representative" of the variety to bag. If you raise only a single plant from the lot, you may or may not grow-out a typical individual. Chances are (statistically) that it will be typical. But it may not be.
Sorry for the long-winded response. Some members may find it useful. Many may not.
Bob