That's not quite right. Your router will be allocating its address for the outside interface based on its MAC address. This will remain the same each time it connects.
The router will also have a different IPv6 address in its internal interface. Hopefully its just taken the first address out of the assigned address space.
Your router will set its own link local IPv6 address, based most likely on the EUI-48 (i.e. MAC address) of one of its interfaces. The RFCs indicate this address should be static, but this is not always the case. The precise behaviour is implementation dependent.
TR-167 allows two possibilities for the router WAN interface's globally routable IPv6 address - SLAAC or DHCPv6. The standard Zen setup is SLAAC, with the DHCPv6 server ignoring globally routable address requests. I'm not sure whether Zen can configure accounts to have a globally routable address allocated to the router's WAN interface via DHCPv6, as I've never asked.
You can, in fact, statically allocate any address from the /64 block to your router's WAN interface, either as the primary address or an alias. My pfSense box has a SLAAC address as the primary address and <prefix>::1 as an alias, primarily for BQM.