It is an evolutionary phenomenon--variation within species (or varieties of fruits and vegetables, or breeds of animals) can occur by natural selection or artificial selection when humans intentionally breed a plant or animal to achieve desired characteristics. See http://www.bbc.co.uk/schools/ks3bitesize/science/organisms_behaviour_health/variation_classification/revise5.shtml

There are well over a thousand types of banana in the world, it seems only natural that the ones that taste good to our palates might not all look the same.
The ones available in the US tend to by the ones that ship well, don't have large seeds and taste "right" to the American palate.