When a flight plan with a request for extra fuel results in a deficit, the fuel deficit only shows what is needed to meet contingency+FAR+alternate, but does not add the missing extra fuel to the deficit.
For example a generated flight plan shows remaining fuel at 58 minutes, but FAR+CF+Alternate = 96 minutes, there is a deficit of 38 minutes which is expressed in kg or lbs.
If 10 minutes of extra fuel was requested, the deficit is still the kg/lbs equivalent of 38 minutes, not 48 minutes.