The new M1 MacBook Pro has "65 (U.S.) or 66 (ISO) keys" plus a Touch Bar.
https://www.apple.com/macbook-pro-13/specs/
The new M1 MacBook Air has no Touch Bar, but instead has 13 more keys, "78 (U.S.) or 79 (ISO) keys including 12 function keys".
https://www.apple.com/macbook-air/specs/
So 12 of the 13 additional keys are accounted for by the extra 12 function keys (instead of the Touch Bar).
What is the 13th additional key (that is not one of the 12 function keys) that is on the new M1 MacBook Air but not on the new M1 MacBook Pro?
P.S. An acceptable Answer would include giving support/documentation for the conclusion that Apple's claim (that the new M1 MacBook Pro has only "65 (U.S.) or 66 (ISO) keys") is an error, that new M1 MacBook Pro actually has 66 (U.S.) or 67 (ISO) keys, and that the only difference in keys is the 12 function keys.
Best Answer
You can compare photos of the two keyboards here:
Here we can see that the two keyboards have the same number of keys in the 5 lower rows of keys. The only difference being that the MacBook Air has 12 function keys where the MacBook Pro has the TouchBar.
The difference in Apple's technical specs I would attribute to Apple describing the power button (TouchID sensor) as a seperate key on the Air, whereas the Pro is is thought as a part of the TouchBar - even though it is physically separate (which is wasn't on earlier models).