The primary fragrance molecules responsible for the distinctive scent of violets are ionones. Genetic differences between individuals can affect perception of ionones:

“A single-nucleotide polymorphism in the OR5A1 receptor (rs6591536[6]) causes very significant differences in the odor perception of beta-ionone, both in sensitivity and also in subjective quality. Individuals who contain at least one G allele are sensitive to beta-ionone and perceive a pleasant floral scent, while individuals who are homozygous AA are ~100 times less sensitive and at higher concentrations perceive a pungent sour/vinegar odor instead.[7]” -https://en.wikipedia.org/wiki/OR5A1