Calorific value

Looking at my monthly bill and working it out I noticed that the calorific value had increased from 39.7 to 40.2? How can this be, surely the only value that increases in working out the bill should be the price per Kw/H all the other values should stay the same?

The chemical composition of natural gas varies and the calorific value changes daily.

See: https://www.nationalgridgas.com/data-and-operations/calorific-value-cv