A few reasons why this could occur.
Just a couple of the top of my head.....
+ Last minute defect that may require a weight reduction due to perfomace.
+ The aircraft may have been over fuelled.
+ A long sector that due to weather/headwinds, was not able to accomodate the usual payload.
Unfortunately, if the loadsheet says your 1 kg overweight, you have to get rid of that 1 kg!
Loose ends are not good!
