Special Fall is an unknown mechanic that, under unknown conditions, increases the drop chances.
The numbers in parentheses in the Special Fall value indicate some unknown trigger condition. What happens after a successful trigger is unknown.
Our unconfirmed theory: the first number in the parentheses of Special Fall is the number of chests opened after which the drop chance becomes equal to the Special Fall value. The second number in parentheses indicates the number of open chests after which the next open chest is guaranteed to give the desired item.
