In my experience it's usually an issue with the underlying data. If you could upload an example file where you have scrambled any sensitive information (Settings > Document Properties > Scrambling) that would be useful in trying to determine what is wrong here.
My guess of the top of my head is that three of the guys with 4000, 2000 and 2000 as their values should be counted twice accoring to what you want to display, but are counted once since they only occur once in the data and thus the total is 8000 lower than what you'd want it to be.
Johannes Sunden is totally right here.
A simple example of this behaviour is the one below: Two products, sales amount for both of them, but one of the products belong to two product groups.
The sum in the source data is obviously 12, but if you sum the rows in the pivot table, you get 17. QlikView, however, shows the correct number - 12.