I seem to recall that the client list in Asus routers is a bit, err, naff. I think I've seen 3 sources for the client name in the list:
a) the clients name set in the client
b) hostname given in the DHCP address reservation
c) MAC address lookup

In reply to a post by jchamier:

Either with Merlin's version or the Asus original, youll find a lot of Asus knowledgeable people on SNBforums.

I've posted there too, no definitive replies yet though...

Purely a guess, but - perhaps VSM Systems wrote the new firmware and it's coming from that ?

"The only recent change to the router was to upgrade from quite an old version of Merlin to the then latest version and the firmware was downloaded from the official Merlin DL site."

The identification is often based on MAC addresses (the hardware address of an Ethernet network adapter). VM's will naturally generate their own random MAC. The same is true of Android phones with Android 9 or 10, as they introduced MAC randomisation. If you have one of these on the network, then that's your answer.