Local factors which influence rankings
1.Search History/Preferences – The fact that Google collects data about almost everything you do on their platforms when you are logged into a Google account (Adsense, Analytics, Webmaster Tools, Gmail and others) it is known for a while, but lately, they decided to become somewhat more transparent. One of the parameters they track is the web search history, namely what sets of keywords you search and probably (or surely) internal data about the results you click on in SERP. As an idea, if you click several times on results from digg.com let’s say, it is more likely that at a future search, which includes a result from digg.com, this will appear above the result from digg.com. This will happen as long as you are logged in your Google account.
Why is this a problem? A webmaster who tracks his results in Google logged in Gmail, for example, will look for keywords which he targets and will click on his own website. (don’t tell me you don’t do this, as I won’t believe you). Storing the result as a preferential one, next time when you look for a targeted keyword on the same site might get a false result.
How to prevent/solve this problem? Don’t make result tracking logged in anything that means Google account. Simple as that.
2. Geo-targeting IP – Apart from the fact that local engines (google.co.uk for example) are obviously based on displaying local results (search ‚seo’ or any other keyword which has a corespondent on google.co.uk and then on google.com), there is other thing involved – a targeting filter based on the IP from thich the search is made. Sometimes, the difference is drastic and again, what you see as a good or bad result, for most of the users may be completely different. Try some searches on google.com with a british or french IP, then set a proxy for a search with US IP. It is very likely to find considerable differences.
Why is this a problem? For a site in English, you could be happy in vain if at your search with british or french IP you are on the 4th position, if for an american you are on 15, for a indian on 15 and so on.
How to prevent/solve the problem? Make tracking with an IP from the country where your target public of your site is. If you make SEO for a site which has people from the US as target public, set a US proxy. If you have an e-shop in French, obviously you are intersted in geo-targeting rank as France, so you need to set the appropiate proxy.
3. Browser – I must be honest, I wasn’t believeing that a simple browser change can affect displaying the rankings, especially since I could not replicate any experiment, each search is displayed to me the same in Mozilla, Opera, IE and Chrome. But I can see how some get different results, especially if using Mozilla, IE, Chrome or Google Toolbar. But this is rather a hypothetical factor.