That is not accurate. I have found that the tracker's info for dark meat (thighs) is sort of wacky and really hard to use properly.
If you google search, you find different info. I'm not sure why spark's tracker info doesnt match this info, and why other sites manage to get it with no problem, but anyway:
1oz of roasted boneless skinless chicken thigh (dark meat) has 58 calories http://www.fatsecret.com/calories-nutritio
1oz of roasted boneless skinless chicken breast (light meat) has 46 calories
So, the calorie difference isn't THAT much, but the fat difference is... 1 g vs 3 g for 1 oz is a lot.
As for why places advertising "all white meat"... well white meat is healthier, but it has more to do with the fact that places like McDonalds use processed chicken product vs actual meat, so saying "all white meat" makes people think they're using actual chicken breasts for the nuggets... when they're not really, they're just using parts of white meat ground up into a paste to make the nuggets :)