Lily ID # 1.5

So I noticed that some of the Corso lilies had 7 or 9 petals. They were appearing on the same stems as your typical 6 petal lilies. I noticed that the ones with 7 petals looked malformed, but interestingly, the ones with 9 looked nearly double. All six petals that one would typically see were present, but in the center, mixed in with the stamens were three extra petals.

I wonder what causes flowers with 6, 7, and 9 petals to appear on the same plant...