Phenotype is genotype + environment. Since environments are so wildly different the same seed will grow differently in various environments. I believe the differences are typically subtle but when you constantly create a similar environment with each generation, the plant will evolve to adapt and thrive in the environment. I think this is why og kush from one breeder to the next has slight variations from one another.
Additionally, since everything is a hybrid anymore, stabilization of the wildly different traits breeders are crossing is problematic. If they don't back cross the newly created hybrid with one of its parents or another plant of the same type as the parents, the following offspring can have wildly different traits.
So, let's say we cross an indica with a sativa. You now have a 50/50 hybrid. These seeds are the F1 generation. They will be relatively similar and uniform. You also achieve what is called hybrid vigor (plants grow stronger and faster) but I digress.
If you took your hybrid and imbred it (which is completely normal in nature) you now have a F2 generation. From what I gather, these offspring can have very different traits of either of their indica and sativa grandparents. They will also be different from each other which can be an indoor growers nightmare.
You also have have to factor in dominant and recessive traits but I'm going to stop there because my head may explode thinking of all the variations and combinations....
This is how I understand it. We need a botanist or geneticist to explain it better....