There are two serial numbers included on your product, one on the headset and one on the down cable.
The headset serial number is located below the side pad cushion on the right side. Gently peel away the cushion to view the serial number. The down cable serial number is located inside the battery compartment and is visible with the batteries removed. Note: The battery door is tethered to the control module. Open it gently to avoid detaching the door. Does either of these numbers match the box?
This is normal! As @Sam_F mentioned the headset and the down cable will have separate serial numbers, so having a different serial number in the battery compartment is to be expected.
I hope this helps!