Smokeless powder production is not an exact science, and many times the industry sets out to produce a large lot of a specific powder, but the end product may have a slightly different burn rate or other characteristics than the narrow parameters of the original. Thus, these powders cannot be considered canister grade for reloaders and must be sold or used separately, but ammo companies have the pressure testing equipment to formulate varying lots and loads using a variety of these powders. The next batch of powder may or may not match the last lot, and the process starts anew.
Ammo companies can easily use these various powders to develop loads, and they can change the loads specifically to the new production variances. Typically, this would not work with most handloaders who expect a canister powder, pick any one, to have very similar characteristics as the last can he/she purchased. However, we all know even varying lots of canister grade powders can perform slightly different, but usually, the differences are slight in order to be labeled as before. I.e. 4350 cannot burn like 4320, Unique cannot burn like Blue Dot, etc, etc