Your body isn't going to add LBM until it feels secure, which means fatter (since fat is stored energy afterall). That's why you need to get to 17...18....19% on "The Program." You need to work through the "growth" range (mid teens). Once you get through that zone, you can dial it back and hover right in that range. But you won't know you were in it until you pass it (assuming, of course, you are an inexperienced lifter).
So yes, it's neccessary to eat that much, to gain the fat to tell your body it's alright to add LBM.