SiteCatalyst does not de-duplicate Orders in Product Classifications. For example:
A user buys 2 products in a single order (A Cheeseburger - $2 and a Hot Dog - $1).
The Products report would show:
Product Units Orders Revenue
Cheeseburger 1 1 $2
Hot Dog 1 1 $1
Total 2 1 $3
Which is correct!
However, if Cheeseburgers and Hot Dogs are classified as Food, then the classification report would look like:
Product Group Units Orders Revenue
Food 2 2 $3
Total 2 1 $3
So the issue here is although Orders are De-duplicated in the totals line, the in-line Orders data is not. This could certainly be an issue, as honestly, there was only one Order for Food made above.
Word from ClientCare is that Discover de-duplicates in-line, but not SiteCatalyst. Would love to see this issue rectified in SiteCatalyst!