It takes several grows of interbreeding several plants to successfully lock down traits. Not all traits can be locked down, so even with extremely good breeders, not all seeds will have the exact same traits.
Perhaps the breeder has locked down flower time, durability in cold temperatures, bud size, and smell, but perhaps they hadn't locked down leaf colour, then each seed may have light or dark green leaves, or even ones with purple (depending on the parent pool), but because the trait wasn't locked down, the colour will be inconsistent.