Except monsters in the Conan RPG with more than 10 HD follow the standard D&D rules and get the same hp they would get in standard D&D. For example, the earth elemental has 20 HD. It gets 20d8+70 hp. (20 x 4.5) + 70.
So, monsters, like you said, dont seem to follow the rules as characters do. They get their full HD regardless of whether its over 10 or not.