When you try a mattress, never press hard with your hand or knee. When you are lying, your weight is distributed and will never be as high, and therefore you will never see what the actual mattress firmness is.
When shopping, a lot of people try the mattress lying on their back. However, most people sleep on their side. Therefore it is important to try a mattress on your side because our weight is different and the contact points are not the same.