In a nutshell, Comcast used to provide a /60 to residential customers and this could be subnetted into more than one LAN. Nowadays they only provide a single /64 and this can only be used for one subnet.
It sounds like your router can request a larger prefix length than /64 and Comcast will give up to a /60. That requires a router that knows how to do that.
That seems like reasonable approach when most people just need /64, and those who want more have to configure to get it.
