different position of a keyword in search results

Hi,
When i search for a keyword in www.google.com.au, the result was found in 1-st page, again when the same keyword is searched in another system of the same network(same ip address) the result was in 9-th page.

What will be the reason for the change of this keyword position.

keywords : online business directory australia , local business listings australia, official business directory australia,

above are the few keywords getting results in 1st page and top position in google.com.au and in some of the system it shows in 9th page in google.com.au.

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Search Engine Optimization|Seo UK

A large number of factors come into play in Google search:
- are you signed into Google? (Different results than when you are signed out)
- have you got the Google toolbar?
- where are you?
- have you searched this term before?

Basically Google is trying to 'help' you as an internet user by changing search results based on your past behaviour and the kind of websites you have visited. It will also assume that you prefer local results.

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Graham and Spence