My brightest keys are 3,4,7,U,F3,F4,B,N from what I can see. This is at lower brightness. The more you raise the brightness, it's less noticeable.
Part of the reason is that some keys have more area where light can pass through. Obviously keys with smaller symbols won't shine through as bright. And since the LEDs are all glowing at the same brightness, you'll get discrepancy in overall brightness. If Apple took it a step further, they could have maybe governed the brightness depending on how much light shown through the key, and adjusted the final output of light.
But that's getting crazy. I have no problem with the light. I'm a touch typist so I don't even look at the keyboard but for maybe a function key or two. I just use the light more as a guide when I go to put my hands on the home row.
Bryan