I'm not sure people expect super-human performance out of Siri. An important difference is that a human who doesn't understand will say so, and ask to repeat the relevant part (or to choose between two alternatives), conversationally; or it will pick an interpretation which is not the intended one but was an understandable misunderstanding.
Contrast this with speech recognition, which will often substitute words that are nonsensical in context, making it look silly from a human perspective...