hmm i have one with a tail just like that and thought the same thing, it was sold as a common but only reached 5" in 2 years, got it when it was about 2"
There are alot of "Common" Plecos now being imported all under the same name..
I don't think anyone really knows what the species are for sure, including the fisherman. They look alot like 'Hypostomus Plecostomus', however, if you look at the Scientific name on the tag of the 'Common Pleco' at your local favorite chainstore (PetSmart) the common plecos now have a new classification.