Two 50gr 777 pellets = ~100 gr Pyrodex/blackpowder
Since 777 is about 15% more powderful (by volume) than Pyrodex/BP, two 50gr 777 pellets is only worth about 85gr loose 777.
They do this because if each pellet was worth the same volume loose powder, people would get confused and drop down 150gr of 777 pellets and have an over-charged gun. Though I'm sure a lot of people don't realize this with loose powder and over-charge anyways.
I don't use pellets, so correct me somebody if I'm off base.