Can anyone comment on the difference between "contributions to total spending in the city economy" and "economic activity generated in the city"? I am curious about the disparity in these two statements:

SF: From April 2011 May 2012 guests and hosts utilizing Airbnb have contributed $56 million in total spending to San Francisco’s economy [1]

NYC: Over the course of a year, Airbnb generated $632 million in economic activity in New York. [2]

Another question I have is how would the authors identify the economic contributions of AirBnB hosts in San Francisco? And/or how they would distinguish between the contributions that were the result of AirBnB and those contributions that would have happened without AirBnB?

I am aware of HR&A's motivations.[3] I am not trying to criticize the report I am interested in the actual analytical process.

[1] http://assets.airbnb.com/press/press-releases/Economic_Impac...

[2] http://www.metro.us/newyork/news/2013/10/22/airbnb-a-huge-bo...

[3] http://www.paulgraham.com/submarine.html