Different google results in different places?

We do a fair bit of SEO work for ourselves and also our clients, so we monitor the search engines and check the positions of our clients in the results when searching for their chosen search terms.

We do SEO work for Wheelworx and they wanted to be on the front page of google when someone searched for Alloy Wheels. We have found that the sites move on a daily basis, sometimes more than once a day. We have had our client up to position one, then a few weeks later down to position 10.

I checked it today, and on my computer they were at position 3. I told my business parter to check and they were 5th on his computer. How can this be?

We both checked the world wise search, and the UK search on google. And both were 3rd on mine, and 5th on his. How can there be such a difference in the results even though he is literally down the road from me?

It makes it very hard for us to do reports for our clients as we are never sure if when they click on the links in the emails they will see the same results we do!

Anyone got any ideas how all this can be?