Why each time the classification model trained by using TrainNewsGroup are not the same?

[Caspar Hsieh]

I use Mahout classification example "TrainNewsGroup" to train the model
with leak type 3, then use "TestNewsGroups" to test the model,
then I re-trained the model and test again, the test result are not the
same before.

why each time the model trained are not the same?

Thanks.