It has something to do with masters used and/or NTSC/PAL conversion... can't remember, but if you are interested, you must do a little google-fu to search in older thread here...
Yes, theoretically PAL should be superior, BUT... theory and practice are not always the same... indeed, even if there are around 5000 PAL laserdisc, only few ones are better than their NTSC counterparts - using similar quality players, of course...