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yoshy-
Is the larger measurement the correction for the bigger rims?
Yes.
Also, they have a nice formula. Generally the width (in this case 40mm) times 2.6 is total width of the tyre when laid flat on the ground ( like this ).
So you can calculate the actual width like this
((Tyre width x 2.6)+Rim width)/π=Actual tyre width (π=pi=±3.14)For example, if your rims have an internal width of 25mm and your tyres are 40x622:
((40x2.6)+25)/π=41mm
Jaap
It's a correction for the generally wider rims used for wider tyres.