Thanks for spotting it.
https://discourse.nixos.org/t/announcing-tuna-nix-mirror/6144/11?u=shiroxia
Please read this post because this is where I found the mirror.
In short,
That is why the signature is the nixos cache one.
Furthermore, the Chinese firewall even tightened bandwidth while accessing github; therefore, I also used a mirror for nixpkgs : https://mirror.sjtu.edu.cn/nix-channels/store