This excerpt taken from the BP 6-K filed Aug 9, 2007.
Note 2 Changes to comparatives
In 2005 the basis of accounting for over-the-counter forward sale and purchase contracts for oil, natural gas, NGLs and power was changed. Certain transactions are now reported on a net basis in sales and other operating revenues, whereas previously they had been reported gross in sales and purchases. This change, while reducing sales and other operating revenues and purchases, had no impact on reported profit, profit per ordinary share, cash flow or the balance sheet.
During 2006, as part of a continuous process to review how individual contracts are accounted for, certain other minor adjustments were identified that should have been reflected in the restatement from gross to net presentation. Though these adjustments are not significant to the group income statement, the amendment has been made to bring the comparatives onto a consistent basis. The comparative figures have been amended to reflect these items as set out below.
BP p.l.c. AND SUBSIDIARIES