There are two separate dists.
This complicates T472 in case we ever want the newer obfs4proxy from TPO's repository installed on Whonix-Gateway.
(Because then we would not only have to add http://deb.torproject.org/torproject.org/dists/jessie/ but also http://deb.torproject.org/torproject.org/dists/obfs4proxy/, download, reupload to Whonix repository.)
Ask TPO if they could upload obfs4proxy to the usual dists (jessie, stretch, etc.) to simplify their repository.