My bag wasn't 25% powder, but perhaps many of them are. Mine was maybe between 15% to 20% fines, not really dust either but pieces too small to use mostly. I dumped it into a plastic container when I opened it so I saw how much there was right away.
Even with 25% dust, that is 30 pounds of usable lump for $13 here. That's $4.25 for 10 pounds of lump. I can't think of a better price for lump that good, and the bags you are comparing it to will have unusable dust/fines whereas the 30# you get for $4.25 is all usable because we already took out the 10# of dust you mentioned.