Long Version: The original Aero mold warped or sumpin'. At any rate, Innova turned it into the Phenix mold and the Aero went OOP. This was in the late 80's.
By the mid 90's there got to be some interest in some discs they had stopped making, so Innova introduced four "Classic" discs meant to re-create some old OOP discs. They were the Classic Cobra, Classic Hammer, Classic Roc and Classic Aero. In the case of the Cobra, Hammer and Roc, the "Classic" designation was important because they still made the regular Cobra, Hammer and Roc. The Aero I guess was called "Classic" just to differentiate it from the originals. At some point they dropped the "Classic" and just started calling it the Aero again.
Short version: It's an Aero made from the recreated mold after they brought the Aero back into production in the mid 90's.