This is because it has been already downloaded/staged on this NS machine.
Example when I'll click to "Download Package" for MS15-80, after that on next right click I'll don't have "Download package" option
If you want to have "Download package" option available for this bulletin, then right click on this bulletin and click "Disable" ⇒ now on next right click you will have an option "Download package" available.
Thanks,
IP.