I can give you a close answer. I noticed today my dhcp leases address 10.0.0.2 - 253, so i wondered where 254 is since the router is 1. I am almost positive it is just being used as a reserve for NAT, i could completely wrong according to "experts" but if i am wrong, then the answer is it is for your home ip phone and it will have a different mac address even though it is plugged into the same router it is just an additional interface. Routers and switches typically have multiple interfaces and they will all have different mac-addresses. Hope that may ease your mind for now. I am betting you might be likely to find your "mystery" mac-address in your connected devices menu option in your router admin page.
network and the mac address on my router dose not match either one my mac add is F8A0977437DF
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