Power difference ? Heat Rate Budget, why?

Hi all,
I would like to understand why, if I assign a "Power Difference" to a BHE through the Borehole Heat Exchanger Editor, that value is never reached in the Heat Rate Budget chart (it is always higher or lower depending on heat extraction or injection).
Can anyone please illuminate me?
Thank you in advance,
Zeno

Of course the BHE as nodal selection with BCs Budget-History Charting Mode...