It is common for a device (or rather, a NIC) to have mutiple IPv6 addresses in different ranges (click for more).
fe80: are non-routable link-local addresses, only valid on the same network segment. Always present.
fd00: is a ULA address, valid in private networks, i.e. potentially for all network segments in your home network. Only present if a ULA prefix is provided, e.g. by your router.
2003: is a public IPv6 address, visible on the Internet. Present if you are connected to the Internet.
Privacy extensions will produce an additional temporary IPv6 address, potentially for each of these ranges.
Pi-hole digests DNS requests, and that protocol leaves only the IP address as distinguishing criterion.
The Network overview, as a relatively recent addition to Pi-hole, is based on link-local network cache rather than DNS, and potentially has access to MAC addresses, but only of link-local (i.e. directly connected) devices.
There are current efforts to associate this to the IP addresses observed, but this is not a trivial matter, as privacy extensions are not the only reason for a device to appear by a different IP address, and then there are devices with mutiple NICs (like WiFi/LAN connected laptops), and devices from other network segments (i.e behind L3 switches or additional routers) showing up under the same MAC (i.e the switch’s or router’s) etc.
On top of this, any solution has to make sure not to compromise Pi-hole’s resolution speed.
Still, I agree that things can get a bit confusing with so many addresses.
At the moment, the only fail-safe way to reduce them would be to disable IPv6 completely (if you do not depend on IPv6 for some reason, of course, and if your router supports it).
Fortunately, your FritzBox is among the routers that would allow you to do so.
Another approach, as suggested here on the forum by e.g. @pisome, would be to keep your FB as DHCP server and distribute your Pi-hole’s link-local address as local DNS server via DHCPv6. This would force your clients to just use their private addresses to send queries to Pi-hole.
You can give this a try, but note that this will only work if your network isn’t segmented (i.e. all devices are connected directly to your FB by LAN or WiFi).