Because it is their job to know.
They are not going to advertise that an animal has genes it does not.
Say I buy a super double sunshine blast for 50k and that little bugger just don't make no rays of sunshine... The breeder is going to owe me some money and I will tell all my friends to stay away from them and they are liars and falsely advertise their product.
I'm not buying a 50k snake from Jim Bob who started producing them last year I am buying mine from BHB because you know that snake has everything they say it does.
Mike