I will give you an example of why there the same, and I might add, I speak from experience on this one.
Lets say Pennzoil/Shell blends a batch of Pennzoil 5W-20 and this batch of oil is 20,000 gallons. First thing they do is load 10,000 in to a rail tank car for a trip to a distributor that has rail service. Now the rest of the 10,000 remaining is allocated for so many 55-gallon drums, so many gallon jugs, so many 5-quart containers for Walmart, and so many cases of quart bottles.
And it all came from the same batch. Now I just use this as an example because usually when they blend a batch of one weight of popular oil, it can be as much as 100,000 gallons.