I think I remember seeing someplace that they are using a non-canister powder, so duplicating (safely) the results with the powders available is going to be unlikely. If that is the case, I wonder who they are getting it from and what all is involved with small ammo manufacturers obtaining it.

I also think part of it is that the recommended loads from the powder manufacturers, especially with relatively low pressure rounds like the .38 Special, are kept on the conservative side to allow for use in older guns that may not be in top condition and/or to allow for unforeseen variables introduced by some handloading practices.