Why do I have an "extra" rib on one side of my belly?

Probably just you. An extra rib in the neck (cervical rib) can be troublesome. It's common to have an anomalous, double, or oddly protuberant individual rib and it's generally of no significance. An 8th, 9th, or 10th rib disconnected from the cartilaginous costal arch can be a minor nuisance but is very common. Breastbones are as variable as noses and ribs also show variety. Variety is cool. Thanks for writing.