Bananas develop a curved shape because of the pull of gravity. They start their lives growing straight. However, as the bunch of bananas spreads out from the top of the plant, the bract, which is the purple part where bananas separate, spreads out and makes the bananas turn upward.
Rather than grow upward, toward the sun, bananas grow away from the pull of gravity. This makes the bananas' growth process a "negatively geotropic" one.
The Cavendish banana is a large type that hangs almost completely straight down from the plant. This position typically keeps the bend in this type of banana fairly consistent throughout the whole bunch. In contrast, Lady Finger bananas sit at almost a straight 90-degree angle to the plants on which they grow. This results in the bananas in the bunch growing differently. The bananas on the top of the bunch grow straight up, while the bananas on the bottom of the bunch grow by twisting around. The twisted bananas are not usually sold in the marketplace, even though they are not any different, except in shape, than the rest of the bananas in the bunch. This is why Lady Finger bananas tend to cost more than other types of bananas.