They may have compressed the PAL version more, they may not have bothered to IVTC when they made the DVD of the NTSC version and wasted a whole heap of space storing the 'extra' interlaced frames, could be other audio tracks, could be anything. I haven't seen the US version so can't really say.
I am amazed at the level of laziness/contempt shown to the PAL regions, a rescale of the NTSC version when they have a PAL master is really unforgivable. I sent mine back with a 'note'.
We often get the rough end of the pineapple in PAL-Land with less extras and so on, but an uprez of an NTSC disc, that is non-anamporphic to start with, that is just pathetic.
Sure the end result isn't totally horrible, but it is way below what a straight PAL transfer would have been. It is like they are *trying* to be bastages about it.