iPhone causing two MAC addresses on wifi

I recently upgraded my Verizon Fios router to Gigabit.  I was adding devices to the new WiFi network and noticed that as soon as I added my iPhone, two new MAC addresses appeared. The first corresponds to my iPhone’s private MAC, but I can’t figure out what the second MAC is for.  It doesn’t appear to be my Apple Watch’s MAC. I also tried turning off my iPhone private MAC to see the underlying MAC, but that isn’t it either. 

I tried changing the WiFi network name and password and each time the same thing happens (albeit with new MACs) when I go to add my phone  

I did notice that this mysterious MAC always has an IP address that is +1 the value of the iPhone IP address. Otherwise, I’m at a complete loss. 

Perhaps it’s related to AirPods case?

Any help would be appreciated!

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