The olfactory bulb also receives inputs from other brain areas, such as the hippocampus and the neocortex, so that the signals it sends to the rest of the brain are already a combination of bottom-up sensory information and top-down processing.
Hence when you smell something, like a live duck in a barnyard or a cooked one in a restaurant, the smell is the result of dynamic interactions of different brain areas involving both sensory inputs and previous knowledge and expectations.