This due to different currency rates being used to show the converted amounts.
If an Opportunity stage is Won / Cancelled / Not Selected any currency conversions uses a dated currency rate, which is based on the decision date. Whereas for the products selections, currency conversions use the current exchange rate. Like below example.... Products Selection: today’s currency rate $->£ 0.603773585 --> £->$ 1.656249999741211
so £165 000 000 * 1.656249999741211 = $273 281 249.9572998
Currency Rate on Opp uses currency on decision date 31/10/2012 = $->£ 0.620713199
£165 000 000 * 1/0.620713199 = $265 823 250.1996466
The difference is due to different currency rates being used to show the converted amounts, the dated currency rate (based on decision date)vs the current exchange rate on the product selection related list.
We need consistency with both objects. We need to match the value of both the field to avoid the confusion for reporting.
Any help on this issue is highly appreciated!!!