The average value of each category is independently calculated, based on the available data we hold. The average total cost might therefore be calculated based on a different sample from the sample that we use to calculate the average values of the sub-categories. For example, a ship-owning company X has provided data for the total crew cost, without providing analysis of this cost into crew wages, provisions and crew other. So, the crew total average is affected by the submission of company X, but the average values of the sub-categories remain unaffected. Therefore, the sum of the crew sub-categories does not equal to the total crew costs. This applies to all categories and sub-categories.