Interesting. My understanding was that it was the opposite; the fine grains would sink down between the larger gravel and push the gravel up. That seems to happen within the varied grain sizes of eco complete on its own.
You are correct. Fine will sink. This is why most avoid capping with fine sands. Eventually the two substrates will start blending and there is a good chance, you won't like the results.
We are talking about aquariums here, not nature. Natural depths of water generate much higher pressures than any of our tank can. The deeper you go, the more pressure exerted on the bottom. I have never been deep sea diving or to the bottom of a river for say but I am sure any sand covered bottom is a cap that is much deeper than our aquariums. Therefore, one can not say the fine sand stays on top. At 1-3ft+ deep, its all on top.