Devices not using Pi-hole

That device is using Pi-hole for DNS.

It is likely that your router is configured in such a way that all the DNS traffic from individual clients is shown as originating from the router, and not from the clients.

Let's take a look at which clients are sending Pi-hole DNS requests. Please post the output of:

echo ">top-clients withzero (50) >quit" | nc localhost 4711