Your own body chemistry affects how different notes react on your skin. Anything that affects the “natural” smell of your skin, such as stress, hormonal changes, your current diet or medications, might change how a perfume smells on you.
Why one and the same perfume smells differently on different people.
A one sentence answer is: because the skin is different.
That is not a very satisfying answer, so let me try to explain. You could even do a science project on this if you have access to a very accurate scale (in the milligram area).
Imagine (or do it as an experiment), you put a small amount of a odorous chemical on a piece of glass and then on the scale. What will happen? The amount of chemical on the glass will get less and less over time until it finally will be gone completely – it evaporates and the scale can tell you the rate of evaporation.
Now imagine, you try a different chemical. In principle the same will happen, but most certainly one of the chemicals takes longer time to evaporate. Now mix the two and repeat. The result will be a mix in-between the two. If you use your nose, in the beginning you will smell the chemical that disseminates more quickly as dominant, later the other one. This happens also every time with a perfume that are usually complex mixtures of 20 and more chemicals.
The “top note” is the first fragrance you get when you smell the perfume. The “middle note” can be smelt after wearing the perfume for a while, so that it dries on your skin and begins to mix with individual body chemistry. The “base note” is the fragrance which lingers. It begins to emerge about twenty minutes after application.
Now back to the experiments:
1) Imagine you repeat the three experiments but you heat the glass by 10 degree Celsius. What happens? The odorants evaporate more quickly, but different chemicals are slightly differently affected.
2) Imagine you wet the glass with water before applying the chemicals. The chemical that dissolves better in water now takes much longer to disseminate.
3) Imagine you cover the glass with a fine layer of fat or oil. The chemical that dissolves better in oil now takes longer to evaporate.
4) Imagine you cover the glass with collagen powder, one of the components of skin. The better the chemical binds to this collagen, the slower it evaporates.
You probably can imagine that skin is a very complex mixture of chemicals. You have water and fat, fatty acids, salts, sugars, proteins, fibers, and hairs. (You probably have heard about different skin types like dry skin, fatty skin, mixed skin.) Each of the components of skin binds the chemicals in the perfume differently and will release it differently. So different composition of skin make the single components of the perfume evaporate more slowly or quickly. Because everybody as a unique skin (if you look carefully enough) for everybody the mixture and amounts of chemicals released is more or less different. These differences are detectable by our nose – the perfume smells differently.
That is in-principle the answer, but it does not even stop here. Change the amount of chemical you apply, for example 2 fold, 5 fold, 10 fold. Or repeat the experiments but cover the glass by one sheet of fabric. Compare the evaporation rates you get on a day with high and low humidity. And finally: the background odors: every room smells different, in a forest the background is different compared to the backyard, to a restaurant, to a shop, to the cinema. These background odorants change the way our receptor cells in the nose respond to chemicals.
You see, it is quite difficult to predict the outcome of the simple experiment: we put perfume on skin – what happens?