Why do perfumes change the way, they smell during wearing?

Let us take a classical example. Your perfume smells like citrus, when you apply it. An hour later it smells, like rose and jasmine, and a few hours later you can enjoy the scent of sandalwood and oakmoss. Fragrances are built of molecules. Naturals or synthetics, the rules are the same - heavy molecules fall to the bottom (will function as basenotes), lightweight molecules jump to the top (will function as topnotes), and the rest will find place in between (will function as middle notes). The more heavy a molecule is, the longer it will last in the scent composition.