What value(s) for CV (Calorific value) is the IHD using ?

I am comparing the data from the IHD (I have beta firmware - but I dont think anything changed regarding this) with what my energy provider gives (EON) for Gas comsumption.

Specifically: the overall usage in KWh, and there are quite some discrepancies: EON reports 67,703 KwH wheras the IHD reported about 70419 KwH for the same day.

The gas volume is however about right (IHD, vs EON reading).

So this suggests to me that the calc the IHD is using may be different - perhaps the CV is different, and the approximation in the IHD is different ?

AI tells me that: "Your supplier, is required by law to use the actual, daily-measured calorific value for the gas in your region. The National Grid constantly measures the CV of the gas as it's distributed across the network...."

wheras the IHD (presumably) uses an standard factor for both temp and CV differences etc.

What actually is the IHD doing ? and why is it so discrpant from my supplier ?