Each garlic clove is a potential garlic plant. When they sprout, it is because conditions such as time, temperature, and maturity level are right for a new plant to begin growing--even if it is not currently underground.
Commercial garlic sold in grocery stores comes mostly from California or China. The University of Maine Cooperative Extension advises that these cultivars (types) of garlic may not be suited to growing elsewhere. Sprouted garlic purchased from local suppliers, such as vendors at farmer's markets, may offer better results due to its acclimation to the local area.
Garlic sold commercially may be treated with special sprout inhibiting chemicals before being offered for sale. These chemicals impede the growth of new garlic plants from the cloves. The chemicals may not be able to stop growth entirely, but garlic grown from treated cloves will be stunted and feeble if it survives.