You are probably looking in the wrong area for the difference. Remember that the PS3 is probably earning EA the most income in Europe, while the Xbox is earning them the income in USD. With the current weak dollar it means EA is probably earning as much as 10-15 USD more per copy of a game sold in Europe compared to each game sold in the US. That quickly adds up.
Also in terms of bookkeeping I would assume the Xbox is treated differently because the business between EA and Microsoft is between two US companies, so they do not need to do any special bookkeeping to keep tabs of fluctuations in currency value. Sony and Nintendo however, being Japanese companies probably means EA gets a lot of their payments in Yen, and since SCEE is basically a independent company the way Sony is organized it is likely that SCEE payments to EA comes in the form of Euros.
