Google Now gets all of its data from the Google Knowledge Graph, which is an artificial intelligence system; as time goes on, this system will evolve and get better. The Google Knowledge Graph was launched in 2012 and actually understands the information that it receives, and how the information can be connected. Google Now also uses the same databases, quite obviously, as Google itself does to harvest its information for searches. Using the Google Knowledge Graph, Google Now is also able to provide direct answers instead of just links to possible solutions.
Siri first searches the web to find possible solutions to the questions that you have provided. However, if it cannot find a direct answer, it then turns to "mashup technologies" to communicate with third-party applications and web services in order to find the answers to your questions or to complete actions that you request of it. One of these third-party applications is Wolfram Alpha, for example. After finding the solutions, however, it needs to convert the information into the natural language and jargon that Siri is so greatly known for--it has to respond like it's your friend. This then returns back to voice recognition, but in reverse; Siri, because it actually responds verbally back to you, must convert its search information into friendly communication, and the produce the sound that you hear as its response.