When your growing from seed , you can have pheno difference.
Like two sisters from same parents one can have blonde hair the other brown. Even if the parents bother have blonde…
It’s genetics, and even so you cold lower your night cycle Temps to try to induce the plant to turn purple, but this only works if the pheno is pre-disposed to turn.
However if you purchased the seeds from Ilgm then I can assure you that you got what you paid for