Andy,
Is this new model? I have not found such model on Varivas internet site? May be you mean Varivas Avani Casting PE Max Power?
Anyway, according to their catalogue, all Max Power and SMP models you mentioned (both jigging and casting types) has the same SP-F coating, so it can not be a difference, I think... And the maker also writes that this line is specially designed for casting (though the material is the same Max Power). So, the question "why is the same material with the same coating different in strength depending on the model?" is really interesting. Anyone has an idea?