This is something that comes up quite regularly. The simple answer is: "the powder they use isn't available to us".
Ammunition factories use what are known as "bulk" powders. These powders aren't the same as "canister" powders that are available to reloaders. When using bulk powders, it takes a full blown ballistics lab to work up the loads, since each lot differs somewhat from the last. They load to specific velocities and accuracy parameters, so one lot may take one weight of powder, while the next lot may take another weight of the same powder.
Canister powders are blended to achieve nearly the same burn rate from lot to lot. There are parameters for each powder, and once those are attained, the powder is considered good to go and packaged for sale. There are variations within those parameters, which is why we're advised to work up loads when changing lots of powder. One lot may be at the low end of the specifications, while the next lot may be at the high end. The difference could be several percentage points, but still within the parameters.
The bottom line is that it's not wise to try to guess what powder is in factory ammunition. Some loading manuals will list a "factory duplication" load, and others will tell you which one was the most accurate in their test bed. Just work up a load for your rifle.
Hope this helps.
Fred
