Answer:

The "Last Scanned" field represents the last time we scanned that subnet; however, an inventory upload needs to occur before the user will see the update in the portal (i.e. it will not be instantaneous). A user conducting a scheduled scan every day on a single subnet should expect that the subnet's "Last Scanned" field to be updated to the most recent scantime, upon the next inventory upload. "Date" represents the last time we saw that Asset, where "discovered" is a tag applied to an Asset when we first "see" it (i.e. inventory the Asset).