If you Google something on your personal computer, and then Google exact same thing from a friend’s computer, you may get a very different page of results. Why is this? Shouldn’t the best results be the same?
The reason the results are different is because Google personalizes search results. That means that your past internet browsing history and your social media history affects the results. So if you tend to click on links from Reddit instead of Digg, Google will eventually give you more results from Reddit.
You can turn off personalized search results, but since it’s an “opt out” feature instead of an “opt in” feature, most people don’t even realize their results are personalized to this. Keep this in mind when working on SEO: the results may be slightly different depending on which user is doing the searching.