I believe lionhead are a genetic mutation that has been bred to stay that way. These are known as hybrid goldfish. It effects their buoyancy and ability to swim normally.
Taken from Aquarium Fish Magazine
Goldfish keepers are attracted to the growing number of "fancy" goldfish varieties: lionheads veiltails, moors, bubble eyes, celestials, etc. Each is a human-engineered hybrid, selectively bred to enhance specific characteristics.
Sometimes grotesque head growth is emphasized, as in many oranda varieties. Other times, strange deformities are enhanced, such as the upward bulging eyes of celestial gazers. Bizarre body shapes may be come fashionable, and so, humped ryukins appear on the market.
There is a price to pay for this genetic manipulation, however. These animals tend to be far more fragile than our childhood carnival pet. They are more sensitive to water quality parameters than ordinary (plain) goldfish. In order to maintain their unique body shape, color and overall health, fancy goldfish require special diets, rigorous tank maintenance and a substantial investment in aquarium equipment.