How the data is stored inside the blendshape node changes because of the evaluation stack. Pre-deformation blendshapes store the corrective blends with the joint rotations undone. To view the raw pre-skinning corrective blendshape in isolation sometimes makes for a very strange looking correction. But once the stack is evaluated it looks correct once again.
In general pre-skinning blendshapes are used by games engines as they are calculated on the CPU before sending the data to the GPU where the skinning calculation is performed. You could perform blendshapes on the GPU but it isn't standard, I've seen blendshapes stored as delta data in textures and then driven by shaders, however this is usually for VFX or secondary shader driven animation.