The borders were uneven, with more on the left than on the right. The idea with the crop and the addborders was to do away with the crushed black and replace it with fresh black, and also to redistribute the black bars better. If you check, it's still 720x480. Coring=False allows for full range. It's all explained in the AviSynth docs if you'd care to look sometime:
http://avisynth.org/mediawiki/Tweak