Why do Additional Costs vary?

Additional Costs are heavily influenced by local conditions, regional price structures and, last but not least, by actual expenditure. It may be the case that similar items have different Additional Costs, e.g. if item 1 was processed with significantly more work than item 2. 
The same applies to completely different items. Occasionally, a supposedly “light" item may have higher Additional Costs than a supposedly "complex" item.