The notification is a summary of the current scan (until you stop it). New devices are ones you've never seen before the current scan. I often run it on my commute, the 'total' number give me an idea of how busy (in terms of BLE devices) it is today.

If you export the database you can pull some more interesting numbers out, e.g. http://www.contextis.com/resources/blog/bluetooth-le-increas...