The sdist command in distutils calculates the MANIFEST file from a template (by default MANIFEST.in). 

The code in sdist assumes that the contents of MANIFEST only depends on MANIFEST.in and setup.py, which can cause files to be missed when building an sdist.

In particular: given a MANIFEST.in file that includes all documentation in a 'Doc' subdirectory, using"recursive-include Doc *.txt" in the template, the MANIFEST won't get regenerated when a new file is added to the documentation subdirectory and hence that new file is not included in sdist distributions.