Google in Michigan

I read on annarbor.com today that Google estimates their ecomomic impact on Michigan is approx. 906 million.  Now, this report is from Google itself.  They arrived at this figure saying their clients get a 2 to 1 return on their investment.  In their "Economic Impact Report" they highlighted their impact on Zingerman's.  According to the report, Zingerman's mail order business reports a 4 to 1 return on their investment.  Now, have you ever been to Zingerman's Deli, Roadhouse or Bakehouse and not seen lines out the doors???????  Have you ever seen them NOT busy?   I realize that the report is referencing the mail order business and not the resturants but Zingerman's was a very successful company to begin with. 
When Google first located offices in Ann Arbor they said that they were going to hire 1000 new employees by 2011; so far they have approx. 200 and 2011 is now less than a year away. 
I in no way mean to criticize Google as I was happy to hear that they were locating a new office in Ann Arbor.  Any new jobs are good for a city.  Two hundred jobs is substantial. 
I don't know much about economic impact reports but do companies usually make their own economic impact reports?  Are there guidlines?  How do they arrive at an economic impact other than how much they pay their employees? 
So far no one has commented on the story online.  It will be interesting to see what people have to say.