It's not the same thing but opposites. Relative density AKA "specific gravity" is the ratio in density of a material and it's environment, when this is not zero then the universe produces forces for motion, just like it does for a high pressure gas that suddenly wants to move to the low pressure region to fill up the space. This is nature's way of keeping things tidy. When something has positive or negative specific gravity then it moves up or down with an acceleration. As its moving up or down this means it imparts a force on any weighing scales so it will show us a weight. Why these things happen is similar to why a stretched rubber band wants to go back to its original position. It wants to be in it's correct place in the universe. If something is less dense it moves up because that's where the less dense things are.