Note that this has nothing to do with objects. You don't seem to be understanding cell arrays.
Your B is a cell array. A cell array is simply a container. When you ask matlab length(B) you ask the length of the container. You've created a container with 3 slots, therefore its length is 3. It doesn't matter what is in the slot, an object, a lump of coal, or nothing, the length of the container is unchanged. Note that {} affects the content of a slot, not a slot itself. You use () to interact with the container itself.
B = {[1 2 3], 'a lump of coal', B_class}; %a container of length 3, containing various things.
B{3} = []; %replace the content of slot 3 with nothing. The container has still length 3
B(3) = []; %delete slot 3 from the container, regardless of what's in it. length is now 2
