Some of the cuttings produced each year are left in their pots for the season, these plants will form little tubers in the pot. Known as "pot tubers". It is a way to ensure that a stock of each variety is kept in safe conditions, just in case there are any hickups when digging up the field grown tubers. These little pot tubers are sometimes sold as such, but the ones kept until the following season can be repotted to make more substantial plants qwuicker.