This particular trinomial name was automatically created by the subsequent discoveries of other subspecies within the binomial name; it is used to set it apart as the original species.
Published scientific literature has an abundance of cases in which species introduced to new habitats have evolved to the point where they will no longer breed with their original species.
In addition, the three original species were phenotypically difficult to distinguish due to different carbohydrate fermentation patterns among strains of the same species.