It is true that if on a given day a person calls in sick and is not replaced we can notionally say there's no cost, although there's a loss of productivity.
In terms of what the total cost of the sick leave is, if we look at the total number of days that are on the books, it doesn't mean that everyone will be taking all of those days, but if you calculate the cost associated with that liability, that's the number that has been booked by the Department of Finance.