That was my first thought too. It's also just as possible that the person that traded it wasn't the first owner, and/or, was completely unaware of it's status. Modern technology being what it is... short of purchasing direct from the mfg... it can be very difficult discerning what is real and what is fake.It might be counterfeit - even with a cage code and serial number. Leopold is very aggressive in regards to this issue, and they will follow through.
Fortunately, with a resource like the internet you can typically find info on how to spot a fake.