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>>>Perhaps I am ranking across all data centres as opposed to a few and therefore more people are finding my site through this particular keyword<<<
In Texas, we call that "hitting the nail on the head".
Does Google use the same search for everyone or do people get slightly different results based on the data centre used?
I guess it does depend on the data centre used.
Just today I did a search on one of my keywords across twelve Google data centres. Whilst my site ranked first position across eleven of those data centres, the same keyword didn't even get me into the top 100 on one particular data centre.