The hashes will only be different if you install different versions of the derivation (package). All the inputs of a derivation, so even selected plugins and co

The hashes will only be different if you install different versions of the derivation (package). All the inputs of a derivation, so even selected plugins and compile options influence the hash. That said, if me and you agreed on a specific git commit of the nixpkgs repo and build some packages from that, our hashes would be the same. This knowledge is also used to download packages from the binary cache. You can see the automatic building of the packages on hydra.nixos.org, from there they will be made avaiable on the binary cache.