The following is known to cause VSE to download the full ZIP file unnecessarily, instead of the incremental files as expected:

• Missing License Information data
  During patch updates or product upgrades, or when using the Repair option, the installation logic to preserve settings sometimes causes the License Information data to be lost. The default location for this information in the Windows registry is: HKEY_LOCAL_MACHINE\Software\McAfee\DesktopProtection
   
• The McShield Service does not load the new DAT file
  This issue is also known as the mutual exclusion (mutex) issue. The McShield.exe process is running but does not load the updated DAT signatures. Instead, it continues to use the DAT files that are in memory.
  
  The registry value AVDatVersion can be found in: HKEY_LOCAL_MACHINE\Software\McAfee\AVEngine
  
  AVDatVersion indicates the DAT signature version that is in memory. The most recent Application Event log source McLogEvent with ID 5000 reports the same information. But, McAfee Agent records that the recent update attempt had succeeded. Despite a successful DAT update (the new signature files are installed on disk and are usable by the managed product), the new files have not yet been loaded into memory.
   
• Insufficient disk space
  The DAT signature file size has been growing in the past 12 months. If your computer has less than 1-GB free disk space, your DAT updates might fail.