setup.py: data_files not doing what I expect

setup.py: data_files not doing what I expect

I have added the following data_files section to my setup.py script:
data_files = [
('libgsync/data': [ 'libgsync/data/client.json' ]),
]
As expected, it copies the file to the following location, but
unexpectedly omits it from the MANIFEST file:
/usr/local/lib/python2.7/dist-packages/libgsync/data/client.json
It also unexpectedly creates the same file here too and does include it in
the manifest:
/usr/local/libgsync/data/client.json
Why? And how do I make it produce something more predictable?
Here's the command:
sudo ./setup.py install --record MANIFEST