You assumptions are only half correct. My mizzen has the same dark material underneath. The main has the lighter material. This was replaced at the time I purchased the boat. Amel Martinique has a special tool for this, a sort of modified jack to lift the mast just a bit for this.
If I were to guess both slabs are made out of solid polyurethane. Apparently the quality of the dark one is better. I may also be that the lighter one is actually an very dense form of polyurethane foam, so it has airbubbles inside contary to the mizzen mast version. Why they chose to change it I don’t know, maybe to dampen the shockloads when in heavy seas.
Just my 2 cents.