Molecules of very different shapes can all smell the same, and molecules of similar shapes can smell very differently. There must be some other difference that the receptors are picking up on. That difference may lie in quantum physics.
Molecules all have a different vibrational frequency, depending on their mass, their bonds, and their structure. It's possible that our noses detect the differences in vibration between molecules, rather than just their shape. There's some evidence for this: A scientist in the 1990s compared the scents of two molecules with different shapes but identical molecular frequencies and found that they smelled exactly the same.