You should never try to compare a web analytics "sales" report to your actual sales report (this is true for any web analytics program).
No, Adobe is not an average... but there is no guarantee that you are going to capture 100% of your sales data for the following potential reasons:
- People actively blocking or opted out of analytics
- Potential JS issues during the collection
- Mal-formed data on the request that can cause the entire request to be dropped
- etc.
Now, you might be wondering why you would track your revenue in your Web Analytics if there is potential to have missing data...
Web Analytics is more about trends than 100% accuracy (this is true for all web analytics data), but also, collecting the data makes it able to correlate sales with other collected information (i.e. campaigns, promotions, traffic sources, what people are doing before or after purchasing, fall off leading up to a sale, return sales, etc).
Reports from different sources will never match, but they should show similar trends, and you can use the information that you are able to collect to better understand what is and isn't working, where there is friction or failures in your website flows, and better correlate return / repeat customers.