Old Dragoon, It seems to me that your BP measurer is set to measure BP ,Like Goex or maybe Swiss. The granulation from BP to smokeless I'm sure is different and may take more or less to fill the BP measurer.
If you used a smokless powder measurer on BP you would see a difference also.
One of the reasons I don't believe that people are getting a true measure useing there BP measurer on the "fake " powder IE- 777, Pyrodex and Clear shot and so on.
I'm not saying my thinking is true, it's just my thoughts on the subject.
A ton of coal and a ton of feathers weigh the same but you can't get a ton of feathers in a coal measurer that's set for one ton of coal
Just another good reson be careful with all the new so called powder on the market today.