Mandragora Officinarum is less common than Autumnalis, and this may be due to it's growing habits. There are fewer locales that it originates from, it is harder to get them to germinate and they don't proliferate as much as Autumnalis does. These factors lend to the rarity in cultivation. (Of course, it's not the rarest, it's just harder and less common.) Secondarily, this variety is a little bit larger than Autumnalis sometimes. Every plant is an individual, so they range in size and shape, but oftentimes, these guys can be larger.
These 3 seeds will ship with a stratification guide to help get your babies to sprout before a year passes. :) (Mandrakes are slow growers and can take up to a year or so to even germinate.) -- That should give you some perspective on the longevity of this plant and the speed in which it matures.