The graph on the left shows how the source-specific atomic weight intervals for hydrogen and carbon atoms affect the molecular weight of the methane molecules they make up.
You can see that the methane molecular weight is lower if the atomic weight intervals of hydrogen and carbon are lower. And, if the atomic weight intervals of hydrogen and carbon are higher, the methane molecular weight intervals will be higher.
Molecular weights have source-specific interval values because the atomic weight intervals used to calculate them are source-specific.
If you are given a sample of methane with a molecular weight of 16.042, do you predict that the sample is from the atmosphere or from fresh water sources?