Paradigms

It was my long-standing belief, arrived at empirically, that all grapefruits had between 9 and 13 sections. That is, until I had two 14-sectioned grapefruits this week.

For a while now I've wondered what controls the number of sections a grapefruit has. It can't be genetic, can it? Perennial crops tend to all be clones of each other within a cultivar. So there wouldn't be any genetic variation at all in the grocery store grapefruits, discounting any theories of a section-number-determining gene or gene family. So what could it be? True randomness? A response to the nutritional status of the mother tree? A function of pollination success? A combination of the last two?

Any ideas?