An explanation from Alliant. It took a while to find.
"We do not schedule manufacture of Promo because we do not "manufacture" it. When we manufacture a batch of powder for a company that is manufacturing ammo we do not always hit their burn speed spec dead on. We can attach a lab report to the shipment that says use 1/2 grain more or 1/2 grain less. It is no big deal. When you buy factory ammo you don't care exactly how much powder is in it. You want to know that it does what the box says. When we make Red Dot or American select, or ..., we cannot put Post Its on each jug that say use 1/2 grain more or 1/2 grain less. It HAS to BE Red Dot or American Select, or ... It is a chemical manufacturing process. Sometimes we end up with powder that is not quite Red Dot or not quite American Select, or not quite... We can blend these batches with other powder to get a specific burn rate. Red Dot has been a 12 gauge target load standard for many, many years. Lots of people use it and there is lots of published data for it. We blend Promo to get the same burning speed as Red Dot. This means you can use the same load data as Red Dot. However, different batches of Promo are blended from different powders. This means the density MAY, or MAY NOT, be the same as Red Dot. When we accumulate sufficient quantities of powders than can be blended to make Promo, we do."
