I could not give you a scientific answer for it, because I am not a chef, but I find that when I am cooking a meat/poultry curry, the oil will not float to the top, until the meat is fully cooked. For a a lamb curry for example, I can tell by looking at it when it is done. The sauce looks all shiny with oil. At this point, if you let it rest, the oil will float to the top.
I do remove excessive oil, but not all. Because a lots of flavour is contained in it.
When cooking the curry sauce/paste, the oil doesn't always separate from the bulk a lot, unless you use as much amount of oil/butter/ghee as chefs do. What you do get is that the curry paste looks shiny and if you stop stirring it, you see a thin edge of oil around it.
Not sure what bajesus means.
"I think the separated oil makes a very large difference in the scrumptiousness in the end."
I agree totally :)