1. The /64 is the same for your whole local network. Granted that at home that is usually not many devices, but it's almost certainly more than one.
2. The /64 changes when you change networks, and unless you have a static IP address it will change for your home network too. On the other hand, if the low 64 bits is derived from your MAC address, it never changes (unless you replace your NIC of course.)
This means that -at best- IPv6 "Privacy Extensions" give advertisers no more information than they get today with non-Carrier-Grade IPv4 NATs. That's not a big win, in my book. :/