Simple explanation:
You have ten people who each own $100 worth of ammunition. When they bought the ammunition, they each paid $100 for it. Something happens and they all decide that their ammunition is not going to be needed soon or worth much so they go to the ammunition market and put their stuff up for sale. But, because the rumors are already spreading about the future value of ammo, they can't get as much for it as they bought it for.
They all end up selling their formerly $100 holdings for $60. That morning, they had in their possession $100 worth of ammo. By the afternoon, they hold $60 in cash instead. Each is $40 poorer and as a whole, they as group have lost 40% of their wealth.
On the other hand, who ever bought the ammo is not any richer than they were before because all they did was to trade $60 in cash for $60 worth of ammo. Their net value has not changed to account for the $40 that simply vanished with the perception of the ammo value.
In conclusion, a lot of the value that exists in the world today is perceived value, not actual value.