Woah... I've never noticed this on my previous 2 Macbook pro's and haven't noticed it with my '12 Macbook Air. But I suppose I haven't really looking for it.
If i'm correct, the MBP screen is a little bit thicker, you can see the same thing but only under really hard sunlight (particularly if your screen is turned off) but yeah I noticed the same thing, on the macbook air it's easier to see.
Woah... I've never noticed this on my previous 2 Macbook pro's and haven't noticed it with my '12 Macbook Air. But I suppose I haven't really looking for it.