Assume,
for example,
the cartons are budgeted at 0.4 per litre.
If we forecast volume of 10,000 litres, the budget allowance for cartons is 400.
If we actually sell only 8,000 litres but use 350 worth of cartons, it is misleading to say that there is a favorable variance of 50 (350-400).
The variance is clearly unfavorable by 30 (350-320).
This only shows up if we adjust the budget of the actual volume level :