My impression is that the super accurate stuff like Google's voice recognition and Siri are all feuled by masssssssive amounts of data. So you build up these recognition networks based off of a bunch of data sources, and get better over time, but the recognition is more based off of the data than the code.

It's the whole "Memory is a process, not a hard drive" thing: Voice recognition as it is today is a slowly evolving graph from input data. You could in theory compress the graph and have it available offline. But it would be hard to chop it up in a way that doesn't completely bust the recognition.