The real reason is because the regular 3.0i is not 1290kg, its actually 1360kg for a manual and 1380kg for an automatic. There are many values online for the weight of the 3.0i ranging from 1290kg to 1410kg. The values I found above are directly from the 2003 manual.
Also I once took my car on a weighbridge and it came out at 1460kg, but included me and about 2/3 a tank of fuel.