If I only upload portal scans that I have never scanned before, why is there a difference between the Stats "portal scans uploaded" and "uniques scout controlled". And why didn't I get to be the controller of all these portals?
To get a point for "uniques scout controlled", you'll need to claim Scout Controller on a portal. For that, you need to upload the most scans to a portal within 30 days. If you scanned portal which are already Scout Controlled by someone else, you need to upload at least two scans to gain Scout Controller.
It seems like the vast majority of portals you have scanned already have a scout controller, so a single scan doesn't grant you the point for "uniques scout controlled"
someone got the controller before you