It does decrease, and it's due to the shape of the flask. Lets simplify it. Imagine that a flask comes to a point (basically, a hollowedout cone with circular base) and is completely filled with an equal parts mixture of two fluids with densities of 1 and 2. Take the column of fluid at the center of the flask. What is the average density of that column? It's 1.5, obviously. Now, imagine that the fluids separate. What is the average density of the narrow column in the center now?
