In flipping through my "Original Challenger and Barracuda 1970-1974" book the photos with ECU's look to have double ballasts. Those would be from years 1971-1974. If you look closely, the photos without ECU's show a single ballast. I even saw a '74 with a single ballast and no visible ECU. There are a couple of pictures where you can't quite see everything but overall that book would suggest that the double ballast was perhaps tied in some way to Electronic Ignition (for some cars at least.)

I know that book is not the ultimate authority and a table from Mopar would be far better but I suspect that Electronic ignition (from 1971-1974) had at least some bearing on the ballast used.