Why is it, that when you buy a glass bottle of full fat milk, there is a layer of cream on the top, but, when you buy a carton or plastic bottle, there isn't?
While both types of milk are pasteurised, the milk in cartons is also homogenized.
Homogenization is a process which breaks down the fat particles to a sufficiently small size so that they remain suspended within the milk, rather than forming a separate layer at the top. Homogenized milk is available in bottles as well as cartons but, for most dairies, the 'default option' is to supply the non-homogenized product in bottles.