2 seeds from the same plant grown under the same conditions at the same time can end up looking different. It's genetics. No 2 things are exactly the same (unless it's a clone).
This.
Its the same deal as your mom and dad having multiple kids. Chances are really good no two of them will be the same and none of them will look exactly like one parent or the other.
Our favorite plant is the same way. Two parents will produce 100's of seeds, and they all will have the same genetics as their parents, but each one of those seeds will be a different phenotype and grow a plant that is all its own.
It is I believe a big reason why bagseed will not necessarily grow dank bud even if the seed came from a bag of dank bud.