One question asked time and time again about SEO and rankings is “Why are my results different than yours?” There is an explanation for this, although it’s not an exact science.
Google Datacenters and Your Physical Location
Google has multiple datacenters around the world, each with tens of thousands of servers. Index changes do not happen across all datacenters simultaneously. Update schedules, crawling schedules, and algorithm change schedules happen at different times for each data center. Crawlers are more likely to index data found nearest to them first. Therefore, index changes are more likely to be seen sooner near the actual location. For example; a change initiated on the West Coast will take a little longer to reach the East Coast. In the meantime, there are index changes happening on the East Coast which also influences search results.
Recent searches and web history also influence the results YOU see. If you search for a keyword when you are logged in, the results may be different from what you would see when you are logged out. A cookie saves your recent searches for 30 minutes in most cases. The reason for that is they are trying to understand why you searched for a particular keyword, entered a URL, or clicked on a particular result in relation to your previous and future search activity. Google tries to provide custom search results whenever possible, which explains why different users see different results.
This raises a few concerns for users and webmasters. Privacy may be an issue to some, but you can decrease the information Google records on you by not being logged in and clearing cookies on a regular basis. The concern for webmasters is that these factors make it more difficult to determine the rank of their website on Google.