Resource Assigments. Actual and Forecast, don't add up to Budget?

Hi

I'm quite new to planning / P6

And have a resource assignments question

I am trying to export the

1. Budgetted Units

2. Actual Units

3. Remaining Units

I go into the Resources Assisgnemrnt Tab and select one of the above and copy the information into excel

However. The units don't add up correctly? I.e

The Actual Units + Remaining Units do not add up to the Budgetted Units figure?

Any ideas?

Regards

Philip