It has to start from the basic building block of life, the DNA. Every gene in DNA comes in pairs. There are several hundred thousand pairs, with no less than millions of base bonds. All aspects of the features shown by the plant are regulated by genes it carries.
When flower is produced, the gene pairs are shuffled. Half goes to the pollen while the other half goes to the ovule. The gene in each seed is recombined from the pollen and ovule. Unless the original gene pairs are identical, which never be, every seed carries different gene set.
That's the reason seeds do not reproduce the same characteristics as the mother plant. Vegetative propagation only involves somatic cells, ie to gene recombination. Therefore, the plants are exactly identical, ignoring occasional mutations.
Hope it helps in the understanding..
