What is the 13th extra key on the new MacBook Air (vs. the new MacBook Pro)?
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).