I think the first year modules for both biochem and biology are the same anyway, so if you get one you can change honors to the other.
To all intents and purposes you are applying twice for the same course in terms of logistics, as such if they give you two offers it will put the estimations of how many will be in the year out.
It would also explain why chemistry and physics is ok, while both are in the same faculty.
As for medicine with maths as a reserve, if the mats dept knew you got an offer for med, or were informed that you had by the medical faculty, why make you an offer when it is obviously a last resort?