Why does curry flavor improve overnight (beyond the effects of permeation and infusion)?
It's down to oxidation reactions that are remarkably similar to those that cause meat and fats to go rancid. From Modernist Cuisine (2-98):


  ...[B]raised and pot roasted meats often develop a richer, more complex flavour if they have been cooled and aged after cooking, then later reheated for service. Surprisingly, the oxidation reactions that cause this flavour-enhancing phenomenon are similar to those that cause meats and fats to go rancid. Although too much oxidation in meat is repulsive, a hint of slowly oxidised aromatic compounds can be quite pleasant.


Also, certain herbs such as thyme and rosemary contain antioxidants that moderate the rate of oxidation thus helping to achieve the right amount of aged flavour, it's reasonable to infer that certain herbs and spices used in Asian cuisine perform a similar function.