I think that because we have to consciously learn to notice and interpret body language, once we have mastered the basics we can pick up on more things than others would, because we've spent more effort practicing to do so. Whereas a NT person might unconsciously have an idea of how another person is feeling through their body language, unless they are incredibly social or intuitive that's all it is--a general feeling. But aspies notice specifics and actually spend the time to think about what it means (obviously, the more practice you have the less conscious the thought becomes, but it's still more so than for NTs). It seems to be particularly true of female aspies, as society seems to push us to fit in socially from a younger age so we are more likely to develop skills like reading body language.
For instance, in my office, I seem to pick up on how certain people are feeling more often than most of my coworkers, because I'm paying attention to body language that others miss, but there is a guy who is incredibly emotionally intuitive who picks up on things I'd never in a million years be able to notice, no matter how much I practiced.