Yep, different on mine too. I don't have the box here, but I recall the SN on the box being A000xxx, while the one the phones is A00xxx, where xxx are numbers, but different numbers on box and phones. When I registered on the sennheiser site, the required format was six numbers I think, so I put in the one from the box. Also, on the phones the format is "S/N A00xxx", that is including the "S/N". But keep in mind that there are two options when registering them: "MX 980 East" and "MX 980 West", so the formatting could be different for them.
Mini review: after using the PK3 and PK2 for a few years, I'm completely blown away by the MX980. The only problem I have is the size of the driver enclosures, which are slightly bigger and don't fit my ears as well as the PK series (The PK series was a dream come true for me regarding the fit, I could wear them for hours and hours without even feeling them). Also, the volume control seems useless. But the sound... let's say I can't listen to my PK2 anymore after using the MX980 for a week. If I can get used to the different size, then this really is the ultimate earbud.