That depends on how you configured your router.
If you set Pi-hole as the WAN/Internet DNS server, all your clients will show up as the router. In this case, this happens because the clients are using the router as DNS server and only the router uses Pi-hole.
There is another way to configure the router, but sometimes this is not possible because not every router allows it: you can set Pi-hole as DNS server on the LAN/DHCP settings of your router. If you are able to configured like this, the router will advertise Pi-hole IP to all clients and the clients will use Pi-hole as DNS server.
Check this post for more details:
Why do I only see my router's IP address instead of individual devices in the Top Clients section and Query Log?
And here you can see how to configure your router (if it allows it):
How do I configure my devices to use Pi-hole as their DNS server? - METHOD 1