I agree with Darth that Chilli, especially fresh, loses heat over a period of time and I think Ashes has hit the nail on the head regarding process. I would add my theory but not necessarily correct; the capsicain oil which is the lethal hot stuff in Chillis changes its chemical structure and concentration when cooked (try deep frying those squat pointy chillis I think they are called Bells - they are delicious and not too hot after frying). When we started serious cooking, e.g. outside catering we had to get the hang of Chillis and what they do. We often found when clearing up after peoples parties that the hot curry which everyone said they would not eat had gone and there was always some mild stuff left. We did our research, and we found out three things:
1. The hot curry was no longer that hot (we used to dip some bread in the bits of sauce left).
2. It was the other ingredients and the richness of the sauce that off set the heat factor, e.g. oil and tomato paste.
3. If we added an acidifier like lemon, tomatoes, or vinegar at the end to crack open the final flavour, the heat factor would diminish slowly over a period of time.
This is why I say never taste a curry when you are making it, learn to use you other senses. I will agre with others who have posted that you can get desensitized during cooking which will misguide you on get the balance correct. I always ask my wife or son to do a taste check at the end as I can gauge their reactions and make any adjustments without tasting.
Just a note on 3. Has anyone posted before on the use of acidifiers? When I was taught, the chef always used an acidifier to bring out the final flavour, whether this be a base or dish.. I was told this was a regular BIR technique as well as a normal approach for ethnic dishes. Maybe this is one of the things you are searching for.
Happy searching
CP