Wax solution

This might be a real stupid question, but it will probably not be the last stupid question I ever ask. What causes wax to rise to the surface of polyester resin? Is it gravity, and being less dense and rises to the top? that is what I have always thought. However, I have hotcoated boards completely around the rails onto the underside, taping off the underside like you would do for a cutlap. The resin on the underside cures normally and is sandable. What am I missing?

The solvent (Styrene) flashes off and brings it to the surface. Also believe it is an oil and water sort of a thing.