99% genetics, 1% technique.
Breeders grow 100s of plant to produce enough seeds for an entire year, the chances of two seeds coming from the same mother plant and are in the same packet is virtually zero.
Anyway, seeds are unique, individual organic beings, not clones and that is why there are differences.
It is a bit like children, even though they have the same Mom an Dad, they are usually very much different, it all comes down to random genetic re-combination.