What causes mis-shaped apple fruit?

Fruit shape is affected by many factors.  Mis-shapen fruit can result from insect or disease damage.  Tarnished plant bug and plum curculio attack very young fruit.  When that fruit matures, it often develops abnormally. 

Probably, one of the most common causes of mis-shapen fruit is the development of seeds within the fruit.  If seed development is dispersed around the core of the apple, usually the fruit will develop well and symmetrically.  If the seeds develop only on one side of the core, then that side will generally outgrow the side with fewer seeds.  This differential in growth results from seed-control of the hormone balance in the associated sides of the apples.  The lack of seeds within a fruit and the asymmetrical distribution of those seeds often occurs from inadequate pollination of the flowers.