I've commented as have others... There is a definite variation in keyboards in the MBA. My Rev A was noticably louder/looser than my Rev B.
I had them side by side and you could run your fingers over the keys and feel that one (the Rev A) was definately looser an louder than the other (the Rev B).
It was a more "clickey" noise..... I thought this was an improvement from Rev A to Rev B until someone else chimed in and said they had very different keyboards (in terms of firmness/noise) on two Rev A models....
There are some very minor adjustments that can be made. I had a stuck <RETURN> key and ended up loosening one of the internal screws. Not that stuck keys is your problem, but it got me thinking that internal screws play a part in the keys and during assembly there will be some changes in tolerence.