For a production series (like the T4, T5, or T6 series,) each weight is going to use the same additive package.
Like said above, there is a limit on phos for the resource conserving weights (10w-30 included) for the API SN, SN+, and SP (also SM, i believe)
If they want to offer a product that is dual rated (between API CK4 and S-), then the formulation will have to conform to both specs according to the most stringent weight,
Shell decided that their T4 lineup should include a 10w-30 weight.
Shell decided that they wanted to continue using an additive package that had >800ppm phos.
Shell's Rotella T4 15w-40 product, specifically that weight, would technically conform to the API SM (and up) spec, because that weight in API SM (and up) has a looser limit on phos.