My guess was that Kodak made a bunch of extra 35mm rolls to somehow avoid the waste of materials on a single occasion, but they knew they could not sell that much Plus-X themselves. A little profit is better than a lot of waste due to a misjudgment. I also thought that the excess material might have come from the MP side of things, and that it was simply perforated for still cameras and sold to Freestyle. Kodak's suggested retail prices for 35mm Plus-X movie film are nearly identical to Freestyle's prices for the Arista Premium 100.
I must say that the new Tri-X is pretty amazing in its lack of grain. I can see why moving Plus-X would be more difficult because of it.