Why does the mattress test ask about my sleeping position?

Because your sleeping position determines which zones of the mattress carry the most load. As a side sleeper, your shoulder and hip bear most of your weight across a small surface area, which means you need a softer shoulder zone. As a front sleeper, you need a flatter and firmer surface instead, to stop your pelvis from sinking too far and arching your lower back. Your height and weight determine the firmness; your sleeping position determines the zoning and the comfort layer — which is why the test asks about both.