Hot Water Return Pump

I have drawn up my domestic hot water system into Hevacomp as a 'flow and return' system with a single line for F+R pipes. When I come to size the hot water return pump it states that the circuit resistance is 20kPa (I have stated the minimum pressure at outlet as 100 kPa within the system marker settings).

However, if I then redraw the system and input a minimum outlet pressure of 100 kPA within the outlet defaults (whilst leaving it at 0 within the system marker) it significantly increases the circuit resistance by 100 kPA.

I'm struggling to understand why this should have an effect on the hot water return pump resistance as a secondary return pump should't be affected by the pressure at outlets?

I was wondering if you would be able to help me by stating the best method to gain a correct circuit resistance for the HWS return pump, as it seems having 100 kPa within the system marker (0 within outlet defaults) and having 100 kPa within the outlet defaults (0 in System marker)provides two different resistance values.

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