We are using Symantec Endpoint Protection 12.1.2015.2015, there are other versions out there, which is what made us look at this in the first place.
So, for some reason the Workstations without AV on them report is: #1 timing out constantly. I'm writing this off on we have over 9k systems that are being scanned and the SQL query is just timing out. #2 Showing over 7k of those 9k systems as not having AV installed on the initial dashboard page. This I know for a fact is, inaccurate. #3 The code I posted below is an edited version of yours, that shows 882 systems, which is very likely and a more realistic number.
SELECT TOP (1000000) tblAssets.AssetName, tblAssets.Description FROM tblAssets INNER JOIN tblComputersystem ON tblAssets.AssetID = tblComputersystem.AssetID INNER JOIN tblAssetCustom ON tblAssets.AssetID = tblAssetCustom.AssetID INNER JOIN tsysOS ON tblAssets.OScode = tsysOS.OScode WHERE (tblAssets.AssetID NOT IN (SELECT tblSoftware.AssetID FROM tblSoftware INNER JOIN tblSoftwareUni ON tblSoftwareUni.SoftID = tblSoftware.softID INNER JOIN tsysantivirus ON tblSoftwareUni.softwareName LIKE tsysantivirus.Software)) AND (tblComputersystem.Domainrole < 2) ORDER BY tblAssets.AssetName