The direction makes sense outside of a gravitational model. More dense stuff holds up/supports less dense stuff. It is why an iron anvil will float in a tub of mercury, which has a higher density than the iron. Gravity (if operational) should always pull a solid through a liquid, as an outside, secondary force acting on these substances. In this instance , it doesn't, thereby indicating the possibility that no secondary force is being exerted beyond the quality and effect of density+volume. The density model is a simple pyramidal model, with the most dense stuff at the bottom. Volume is also at work however, as you need the volumetric force of one object to exceed that of the other. You need a certain volume of mercury, for example, to resist the volume of the iron anvil.