According to Google, they update things almost every day. And you should not see any big changes. If you do, it just is what it is. Meaning – nothing special. Things will always fluctuate. The whole world is fighting for the top spots.
As long as you make good content for people and don’t mess up in any way (and don’t try to trick the search engines), you will do ok. And if anyone say they can get you to number one – run away. Fast.