PHPP slab on grade U value vs floor component U value

[frankquietly]

Hello, Please could someone spare a moment to point out why I'm a plonker?

 

I've calculated my sub floor U value as 0.6 using the BRE perimeter:area method.

 

 

 

 

This has been entered in the PHPP ground sheet

 

The floor component has been setup with variants so I can model floor improvement

 

 

 

 

However when I change the insulation values, PHPP recalculates the floor losses so they always stay the same. It seems whatever it has decided is the monthly heat flow on the ground sheet will carry through to the heating demand, regardless of the interior insulation value of the floor component. So when the U value changes the "Gt" in the heating sheet adjusts to keep the kWH/a the same

 

 

 

Note I am only doing PHPP for geeky interest while I renovate my terraced house and would appreciate if anyone could help me understand.