Assumedly the mechanism at work is like any fat (oil and water don't mix) it simply does not bond or has a density and saturation level that's very low and weighs less. It's actually a handy way to lower the fat content of your soups. But here's the scientific skinny ( a little anyway). The congealing and thickening aren't really talked about but then again, that's notwhat you asked! PS the gelatinous mixture is the whole game, I prefer a thick richer stock over a consomme any day.
Fat is dissolved by boiling; but as it is contained in cells covered by a very fine membrane, which never dissolves, a portion of it always adheres to the fibres. The other portion rises to the surface of the stock, and is that which has escaped from the cells which were not whole, or which have burst by boiling. Since the fat is less dense than the broth, it rises as the soup cools and hardens.
Gelatine is soluble: It is the basis and the nutritious portion of the stock. When there is an abundance of it, it causes the stock, when cold, to become a jelly. Osmazome is soluble even when cold, and is that part of the meat which gives flavour and perfume to the stock. The flesh of old animals contains more osmazome than that of young ones. Brown meats contain more than white, and the former make the stock more fragrant. By roasting meat, the osmazome appears to acquire higher properties; so, by putting the remains of roast meats into your stock-pot, you obtain a better flavour.
Fat is dissolved by boiling; but as it is contained in cells covered by a very fine membrane, which never dissolves, a portion of it always adheres to the fibres. The other portion rises to the surface of the stock, and is that which has escaped from the cells which were not whole, or which have burst by boiling. Since the fat is less dense than the broth, it rises as the soup cools and hardens.
Gelatine is soluble: It is the basis and the nutritious portion of the stock. When there is an abundance of it, it causes the stock, when cold, to become a jelly. Osmazome is soluble even when cold, and is that part of the meat which gives flavour and perfume to the stock. The flesh of old animals contains more osmazome than that of young ones. Brown meats contain more than white, and the former make the stock more fragrant. By roasting meat, the osmazome appears to acquire higher properties; so, by putting the remains of roast meats into your stock-pot, you obtain a better flavour.