An offset (branch) is not an exact clone of the mother plant. It could be size, color, variegation, bloom deviation, etc. It could be caused by a virus, genetic mutation, or some environmental reason as to why it sported. Without sports the plant world may be pretty dull and boring.
This is true in all plants. For instance a variegated plant might be a sport of an all green mother plant. If you have a variegated plant you should trim any branches reverting back to its original color (usually green) since this is more dominant and if left unchecked may eventually turn the entire plant back to its original genetic makeup.
Tulips have many sports in blooms and in foliage for example and may be the most used plant in the world when it comes to sports. A lot of variegated shrubs are sports.
An enthusiast might take notice of a sport, cut it, propagate it again and again and introduce it to the trade as something new. A lot of sports go unnoticed since they occur in nature often and most folks see no value in it.