This can happen if you have devices that were just pinged, and not really scanned in any way. If your scanners hit targets that give very minimal response, so much so that we can't actually run any port checks against it, 19506 won't fire for that IP because there weren't any plugins that were able to run against it that would trigger a vuln scan worthy of notating. Often times this is caused by host discovery scans being run on certain subnets, and no followup vulns scans were then run again on those same targets.
You can create a dynamic asset list that has "Plugin ID is not equal to 19506" set as a filter to get all of the hosts that fit that criteria. You can then use that asset to scan those IPs to see if network conditions have changed enough to let your scanners hit them.
Also, you'll sometimes get firewalls or routers that respond on behalf of any IP that fits within their assignable ARP table, regardless as to whether or not there's anything actively plugged into those ports or assigned those IPs. Usually this feature exists on those devices for faster network speeds and address assignment, but it will cause our scanners to get a response from something that doesn't exist. The good news there though is that they don't count against your license, as we don't count certain basic info plugins like ping against your host count.