There are several possible explanations. Since it occurred and since we do not have parentage information in the registration databases the simplest assumption to begin with is that Lavender Stardust has the one allele to produce purple and one to not produce purple - it is heterozygous. We would assume that it cannot produce reddish pigments at all. As normal then Lavender Stardust is Pp. Each time a cell divides it doubles its DNA first and then shares half of that DNA with each daughter cell. So Pp becomes PpPp and each daughter cell gets Pp. Sometimes a mistake happens. One mistake is that when the DNA is doubled one of the P's becomes a p. Then one daughter cell is Pp and the other is pp (not lavender). All the descendents of that cell will be pp and not lavender (lets assume that they will be near-white in this case).
A whole flower probably has hundreds of thousands (or more) cells but flowers typically start development as a handful of different cells. We could assume that this specific flower started its development as three cells. When they doubled one of the cells had a mistake so one of the six daughter cells was pp. All the daughter cells that were produced in that lineage were pp and they were located in a position so that they formed one half of a sepal and one half of a petal of the final flower when the flower had finished developing.
your poly!
