Any number of things could cause this behavior – for EXAMPLE:

1. The asset has been deleted and ignoredeletedassets is set to 1 in the registry (HKLMSOFTWAREBigFixEnterprise ServerAssetDiscoveryNMAP)
2. The asset has become managed, then unmanaged from scan to scan.
3. FIREWALL policies changed on said workstations from scan to scan.

If you scan one asset that is unmanaged – that asset will be stored in the DATABASE and appear in the unmanaged assets table. If you scan the same asset when it is down (e.g. that IP is unresponsive to ping), it will still persist in the unmanaged asset table. If the asset is scanned once again, it will again, still remain in the table but have an updated last scan TIME.

Any number of things could cause this behavior – for example:

If you scan one asset that is unmanaged – that asset will be stored in the database and appear in the unmanaged assets table. If you scan the same asset when it is down (e.g. that IP is unresponsive to ping), it will still persist in the unmanaged asset table. If the asset is scanned once again, it will again, still remain in the table but have an updated last scan time.