Google is constantly updating their results. There is not just one index that everyone uses, and they have multiple locations with hundreds of thousands of servers. As the changes to the results are made, they need to propagate out to the other datacenters and indexes.
From search to search you may end up on a different index, with slightly different results. There are also customizations based on localization (where you are), personalization (your Google account history) and things like the last things you were searching for may also become more prevalent.
In short, the old concept of 1 single set of rankings that's updated every so often has long been dead, and there is now a fluid and ever changing cluster of indexes that increasingly will not serve the same results to different people.
Every now and then an algorithm change will shake things up and look like the Google updates of old, but the index is now updated all day long every day as they crawl and isn't updated once a month like they used to back in the day.