Anecdotal counterpoint: Siri is horrible at recognizing my speech if it is anything that it doesn't expect. I can ask it to define a word all day long but I doubt it will get any better. It just keeps giving me the same (incorrect) response repeatedly. Google Now does a very good job at recognizing voice including words I may mispronounce when I say "define X"
I can sort of imagine why. Google "knows" me better. What is awesome/scary is that I have tried this on other people's iPhones and Google Now still does better than Siri.