This has popped up before, put I just had a look around to see if it's being discussed lately and I don't think it is. I always remind myself of this, to make sure I'm not eating empty calories.
The scores are explained here:http://onthelime.com/_blog/On_The_Lime_Blog/tag/kale/
But basically, they're saying per calorie, Kale and Collards have the highest amount of vitamins and minerals. Relative to the amount that they have, they then have scored other foods per calorie.
It's because of this that I eat so much spinach now, I eat it every day, because I can get it all year round. Now that it is Winter, I can get Kale, so I am eating a tonne of that.