That rule of thumb was for lead shot sizes #6 and larger. As the lead shot size gets smaller, the weight differences become less. For lead #8, size seven steel may be lighter or heavier depending on the actual alloy. This is particularly true when comparing high antimony lead shot to a heavier alloy steel shot.
In BB steel, I have seen as much as a 5 pellet discrepancy in a weighed payload between different bags of shot whether different manufacturers or same but different lot. That inconsistency is one of the reasons I stopped loading steel.
