Well, yes, a gram is a gram, but people ignore the inherent inaccuracy in weighing out some dry chemicals. The problem is not the precision of the scales, but the water content of the chemical being weighed. Store some anhydrous whatever for a while and it will inevitably absorb some water, sometimes lots, over time. A gram of the older, water-laden chemical contains more water weight and less actual reagent than a newer, completely desiccated batch would.
To be sure, this only affects some chemicals, but it's an issue nevertheless. To reduce the margin of error, I try to buy chemicals that are in their most stable state; neither tending to absorb nor lose water over time with storage. Keeping things in tightly sealed containers helps too.
Best,
Doremus