Ah, store-paths.xz.
As I maintain the TUNA Nix mirror (I maintain the download script, TUNA provides the server), I also need to copy entire channels. I have also used store-paths.xz but as you have seen it only includes almost every path, not really all of them. For this reason in the help page
It seems that store-paths.xz is generated by Hydra, corresponding to the jobs of a channel, and posted by this script, whereas cache.nixos.org consists of every(?) store path Hydra produces.
I haven’t dug much further than that though.