More specifically, the tarballs that Kovid distributes from the download page are more than just the source code from version control (which is what Github's autogenerated archives will get you).
The tarballs on the calibre download page (and also mirrored on Fosshub and,
for the latest release only, on Github), contain a number of build artifacts, for example, the compiled localizations, the transpiled rapydscript-to-js code for the server, etc.
You can create all this yourself, by using:
Code:
python2 setup.py bootstrap
This is what I do in my Arch Linux build recipe for the
calibre-git package, which, since it builds from a git checkout and always uses the latest git master, cannot rely on the pregenerated release artifacts in a release tarball.