Because some ISP will provide only a prefix, for example my ISP is exactly doing the same as OP, which gives /56 prefix without address, so in this case WAN6 doesn't need an address, the most important thing: /56 prefix already there, and the /128 WAN address is just a by-product of using DHCPv6. In fact OP doesn't really need to add "ip6prefix" line in this case due to the presence of RA.