The hardness of the Dunlop ball is exactly what that shock (& vibration) transmitted the arm is. What other explanation would you have for your ability to feel the hardness/stiffness of the ball? As for heaviness (static weight), the balls are within ITF specs (which are fairly narrow as I indicated previously). If the balls really are heavier, on average, that other balls that you use, the difference is, no doubt, extremely slight. It is doubtful that you would even be able to detect the difference with most commercial scales (like a food scale or a postal scale). A ball may give the illusion that it is heavier (weights more) because of its stiffness &, perhaps, due to it's dwell time on the stringbed. .