GRASS in Debian
Grass in Debian is maintained by the DebianGis Team. Some modifications to the code base are due to the Debian Policy compliancy required by the distribution. You can find that the Debian stable version is generally older that the current stable version of Grass: that is due to the releasing and maintainance roadmap of Debian.
Unofficial newer packages for Debian stable can be available by third parties. Generally you can mix those packages with backports.org packages in order to work on a more up-to-date workstation, but the result can be less stable and coherent than the official release, because they are generally built using unstable snapshots and sometimes packages are not compatible each other. This is due to the binary nature of the packages and policy evolution.