I bought my first air 13" last month. It works awesomely and I really love it, but there is a little defect on the screen frame, so I called Apple to return it and get a new one. Today I received a newly made one, so before Apple retrieves my old air (not really old, it has just been used for a week), now I have both machines. This is a terrific chance so I made detailed comparisons between them, and I found out something interesting. These 2 MacBooks should be identical, but actually they aren't. First, the image qualities of 2 screens are slightly different. The old one is more saturated and more pleasing to my eye, while the new one is little pale. My guess is that the old one was calibrated and optimized more. Then, the sounds of typing on both keyboards are not the same. There is a high frequency sound from touching keys of the new one. After I "opened" up the Command key of each machine, I found out that the underneath mechanism is also different. The old one has 1 curved metal bar for the key to attach on, while the new has 2. That explains why the sounds of typing are different. While they both are well made machines, I am very curious about the differences. Is it usual? I start to wonder if my old one may be actually a refurbished one..