It does seem odd that there would be that much variation in screen calibration/quality, but I have seen stranger things....
The screen on my 15" PB is beautiful. Absolutely no complaints, no color cast, very nice detail for the default resolution, as nice as any laptop I have ever seen.
I have a couple that have the UXGA, or WUXGA display that are sharper, but the tradeoff is a screen that's so huge, I can't see anything on it without a magnifying glass.
If the displays are the same physical size and resolution, then they are most likely from the same manufacturer or equivalent. Typically cost forces them to only substitute displays for a reason, such as higher resolution, or some kind of other selling point such as Sony's "x-brite" or Toshiba's "Tru-brite". I may have those backwards.
Usually they will call out differences like that in thier spec or literature and I can't seen anything in the PB stuff that mentions a better screen.
Strange.....