Data loading from DSO to Cube

Hi,

I have a question,

In book TBW10 i read about the data load from DSO to InfoCube

" We feed the change log data to the InfoCube, 10, -10, and 30 add to the correct 30 value"

My question is cube already have 10 value, if we are sending 10, -10 and 30 Values(delta), the total should be 40 instead of 30.

Please some one explaine me.

Thanks