The question should be why ONLY two. I get 2^72 addresses from
mine. Yep, a gazillion addresses! Even my cell phone gets 2^64
addresses. As for two, check again tomorrow and the next day, etc.
If they're using SLAAC, you get in addition to a consistent address,
up to 7 privacy addresses, with a new one every day. The global
dynamic address is your consistent address and may be based on your
MAC address. The other one is a privacy address. Are you
connecting directly to your ISP? Or through a router that supports
IPv6? If a router, you may have more than 1 /64 available. I have
256. You can split off the /64s to different networks. For
example, I have 1 for my guest WiFi, in addition to my main LAN.
BTW, I run pfSense for my firewall/router and it supports multiple
/64s. If my modem was in gateway mode, I'd only get a single /64.