I'm really surprised that no one has hit the nail on the head with this. There are different varieties of brightleaf, and the one that we are most familiar with is VAGold25 seedstock. We (pipesmokers) tend to call all brightleaf varieties, Virginias. But, this is not so on the agricultural end of the varietal. There are a few varieties that were cultivated in Africa and Asia that will wallop your head... particularly African Virginias. It is not a brightleaf, like we would call a "Virginia", but it is of that strain. And, there are others, in a category we (pipesmokers) would call Imperial Virginias, referring to those tobaccos cultivated by the British Empire, when they had one. And, it is these Imperial Virginias that give Royal Yacht its strength.
Also, yes, the topping changes the acidity of the blend. Virginias naturally have a wallop of nic power, but they are acidic (PH-) which makes it harder to absorb the nicotine in that leaf. And, a topping that changes the PH can make it a more powerful blend. But, I am not sure that is in play here... maybe.