I understand your point, but these aren't "two oils" any more than a PYB 5w-20 is a different oil than the PYB 10w-40 you find on the shelf. You have to pay attentions to specifications. In the PYB case, the former is SN/GF-5 and will have low phosphorus by nature of its specifications. The 10w-40 may or may not have low phosphorus.
The same goes with this. Because of the E6 specification (not to mention the loophole closing around CK-4/SN in 0w-30, 5w-30, and 10w-30 HDEOs), the 5w-30 Rotella will be low phosphorus, and the other stuff, like the 15w-40, may or may not have low phosphorus (we here know it doesn't, but the average schmo won't).
And you're right, the average schmo could be a bit confused, but that's nothing new with oil specifications. As always, it's best to follow the OEM recommendations, and only go off that when you actually understand what you're doing with the specifications.
Nothing on the 5w-30 T6 bottle is misleading, of course, and the same goes for the data sheets. The builder approvals are all there, and the API and ACEA specs are listed, not to mention the SAE grade.