That was probably my fault.
But now I'm confused. Is it the backlight that doesn't have the same intensity? Or is it the pixels?
The contrast (of the pixels) is controlled via, uhh VO. (VEE on other LCD's, had to make sure, lol) Have you tried adjusting the trimmer pot on the top left of the shield? That's what controls the Contrast.
You should try connecting the backlight to 5v instead, and play with the contrast, check that to make sure. 
But that's pretty nifty the backlight is controlled by a transistor for you.