"Chop" suggests the pieces are chunks (even when "finely chopped"), but shred means the pieces are very thin and long (but not julienne). If you look at images for "chopped cabbage" and "shredded cabbage" you'll see they look quite similar. That's just how cabbage goes. However, images for "chopped carrots" and "shredded carrots" show a very clear difference. Image searches help a lot. :)