Smell is the only sense with a direct line to memory. Sight, sound, touch and taste all get routed through a part of the brain that processes and files information first. Smell mostly skips that step — the olfactory bulb sits right next to the amygdala and hippocampus, the two structures most involved in emotion and memory, with very little standing between them.
Why a smell can undo you faster than a photo
This is why a specific smell can put you back in a specific room, at a specific age, faster and more completely than a photo of the same room ever could. Psychologists sometimes call it the Proust phenomenon, after the writer who spent several pages of a very long novel describing exactly this happening over a biscuit dipped in tea. The smell doesn't really remind you of the memory. For a second, it's closer to being the memory.
What that means for how a fragrance gets made
It's why a scent that's technically "correct" — balanced, well-blended, pleasant — can still feel like nothing in particular, while a slightly rougher one that happens to smell like somewhere specific becomes someone's favourite within ten seconds. Nostalgia can't really be manufactured to order. All you can do is make something honest enough that it has a chance of landing on somebody's actual memory, rather than smelling like nothing at all.
The unhelpful part
There's no way to predict which memory a given scent will hit, or for whom. The same bottle that means nothing to one person will stop another cold in a doorway. That unpredictability is more or less the whole reason to make four different scents instead of one.