These are called Ghost Updates. If you get one it can mean one of the following for you:
1) VO is working on your file and made some changes which won't reflect on GC key for now but would later on.
2) Its probably just a system refresh or something of that sort. You may call it a glitch if you like.
The first one, in case of complete applications, as you may find in this forum, is often followed by a final decision after a couple of days. But not guaranteed though.
Just keep your hopes high, you might hear a good news soon.