Ian;
I think the answer is pretty simple: Vaillant doesn’t quality-control the flow and return temperature sensors. It’s random whether you have a good pair or a bad pair, and a bad pair can read up to 1 K different, which is the difference between a CoP of 2 or 5 – neither value being correct.
There’s a whole thread about it; I documented my experiences with fixing this on my Arotherm in that thread beginning here.
Edit: which Andre K said in the first reply 
