My understanding of nix channels is pretty limited but I think I may know the answer here.
Each nix channel is checked out on NixOS’s build servers. The servers compile the software in the channel and run tests. If the tests succeed (and at least if certain set of the software in the channel compiles successfully), the channel is published and everyone’s machine gets it.
Basically, the git may have version 26 in the master branch, which the server checked out and started running tests. Let’s say that hypothetically GCC is broken in the master branch of nixpkgs, so the entire branch is rejected and the old version remains as the “unstable” branch. This will continue until GCC is marked broken or is fixed, which will then allow the tests to succeed and will let nixpkgs update.
IDK if this makes any sense or is even correct. Hopefully someone knowledgeable can explain it better.