The flavor of a curry usually improves overnight. Harsh overtones mellow and disparate elements combine for a richer, more coherent result. Some people think this is just because the flavors from whole spices have more time to infuse (and larger chunks of vegetables and meat exchange flavors with the sauce base).   However, I think there's more to it than that. Even with a curry without large chunks of food or whole spices, the flavor still seems to improve over time. Is this really so, and if so, why? 
In addition to oxidation as put forward by @Stefano, you also have    Slow chemical reactions between compounds. Oxidation is only one chemical reaction that happens, there are many more Concentration of flavors due to the evaporation of water will give a more intense flavor  