The Why Microsoft blog has an interesting article that details the difference in how availability is calculated in Microsoft Office 365 and Google Apps. The article focuses on seven important criteria in considering cloud service availability, and compares how each is calculated in Microsoft Office 365 and Google Apps. The seven criteria include: 

1. Percentage of users affected by an issue  
2. Which customers are used in calculating availability  
3. Services reflected in the availability calculation  
4. Number of cloud services covered by the Service Level Agreement  
5. Number of cloud service outages across a range of providers  
6. Amount of resources the provider invests in delivery  
7. Amount and timing of customer remedy for any lost productivity  

In my opinion, the calculations used for Google Apps Availability are heavily skewed in Google’s favor, rather than valuing the customer. 
 
 
The article can be read here.