AirPods (gen 2 only) mismatched serial numbers?

What is the reason for a pair of AirPods to have separate serial numbers? Why only one matching what's on the box as well as the case? How is the second serial number relevant to anything, and how can you identify its matching serial number it pairs with from first purchase? Perhaps it's just me, but I'm having a hard time finding a way to wrap my head around this.