Does your ISP sell static IPs? Maybe they are all static?
For an ISP using all public IPs, in the days of dial up they could rent less IPs than customers because people were online at different times. These days the routers are all online 24/7, so it seems odd to me that some ISPs have everyone on public IPs but they aren’t static. Probably some technical reason why things don’t work how I think they do, but it just feels like a way to sell static IPs as an add-on when it wouldn’t cost them anymore to allocate an IP to a customer for the life of the connection.