TSHVector 3th Order Coefficients

I am trying to update the Irradiance Map from a SkyLight through the USkyLightComponent.
I`ve found that a TSHVector<3> have a variable V, which is a vector of 12 floats (SH coefficients).

This seems a little odd since 3th Order SH have 9 coefficients. What are those extra 3 floats from this vector?
I could not find any information on this in the documentation, besides that this number comes from NumSIMDVectors (3) * NumComponentsPerSIMDVector(4).

It’s probably in XYZW format. That would explain the extra three floats.