Your router may distribute its own IPs as DNS server, usually because you configured it to do so (e.g. by setting it as secondary DNS server), or because it is offering its IPv6 address.
There are also some routers (like this ASUS RT-AC68) that distribute themselves in addition to any custom DNS server, no mattter what you configure.
You may try to verify this, e.g. by running the following commands from a Windows client in your network:
netsh interface ipv4 show dnsservers
netsh interface ipv6 show dnsservers
If any of you router's IPs shows up in the results, your clients may use your router as DNS server at their own discretion.