Powder manufacturer try to maintain a consistency of charge in their canister powders, (What we buy over the counter).

As this was explained to me, they blend the powders, so that a charge of a Brand of powder you use will have some consistency to it no matter when you buy it. Supposedly it will have the same burn rate and pressure from batch to batch.

This is analogous to Single Malt Whisky distillers blending within a particular Single Malt to preserve and maintain a certain taste or quality.

Non-canister are used by manufacturers and they simple vary the charge of powder from batch to batch to maintain a velocity and pressure for each cartridge they manufacture.

I would not be surprised in the Federal rounds having a different powder charge depending on the lot or batch.

I think this explains the basic difference.