Hi. I grew up with IPv4, so IPv6 is a little confusing for me. But I try to explain what I see:
My router shows me some IPv6 addresses for the same device, and I can see some of them in the tools -> network listing of the pihole. But nearly every day there is a new IPv6 I never saw before and I cannot find on my router.
The pihole tells me that the device connected to this IPv6 is my pc (for example). But I cannot find this IP anywhere on my pc (netstat, ipconfig) or the router.
Sadly the pihole doesn't show all IPs of every device (after 3 or 4 it just returns "...").
How does this work? Where do these IPv6 come from? Why do my router and the pihole differ from each other?
I'll try to explain your observations for Pi-hole - but explaining IPv6 is out of Pi-hole's scope.
I''d recommend to put some time away for researching and investigating about IPv6.
Just know that your observation seems not uncommon.
Your ISP may provide you with a new IPv6 prefix regularly at its own discretion, unless your contract says otherwise. Regular changing your IP address assignment (both IPv4 and IPv6) may even be a regulatory requirement under your country's jurisdiction, in an effort to increase your privacy by making it harder to track your IP.
With IPv6, a change of a prefix would prompt your client's to construct or request a new IPv6 address from that prefix, by combining it with an interface identifier. Independent from prefix changes, clients may also decide on their own behalf to use new interface identifiers under a number of conditions, e.g. when using RFC8981 Temporary Addresses
Pi-hole reports IP addresses as historically observed on its network link.
An IP address that has been reserved for a client or that a client claims for itself, but that was never used by that client to handle traffic on the same link that Pi-hole resides on, cannot be observed by Pi-hole.
E.g. this could happen if your router reserves an IPv6 address within its DHCPv6 server for a client, but the client would only use addresses acquired via SLAAC, using Stateless DHCPv6 or NDP to configure itself for your network. Or a client would have created an IPv6 ULA address as suggested by your router advertisements, but never use that address.
For clients with changing IPs, Pi-hole's network overview may well list an IP that is not in use by that client anymore.
And as long as IP address assignments are non-static, Pi-hole may observe the same IP address in use by several clients at different times, e.g. if your router's DHCP server would hand out the same IP to your homeoffice laptop in the morning and to a smartphone in the evening (when your laptop is off).