The figure on the exchange is impossible to know for, obviously, many people hold coins there with the purpose of selling the at some point but not necessarily listed for sale right now. As a matter of fact, less than 200k are listed for sale at all kind of prices. The figure in the Maximum network weight can be misleading for several logical -and others than I don't understand- reasons. For instance, although all my coins, are mature, my network weight is roughly 15% below the coins I actually have "staking". I would imagine that the maximum network weight of 0.5 mill would also be off at least by 15%, probably more. One of the reasons that I understand is that the mined coins -and it is a significant number- are not mature until 4 hours after being mined; other reason would be the coins in movement towards the exchange and from the exchange. In any event the maximum network weight oscillates between low 400k and low 700k during the day, which is a big oscillation and the reasons for it could be a combination of all of the above. I don't suppose -but it is always possible- people will already have significant amounts in cold storage, not staking, but whatever some people has, it would also contribute to the difference.