Different SERPS depending on page settings?

Preferences set to 20 differ from those set to 100

[netmeg]

I have a client who has a new product, and wants me to optimize a page for that product. I went to look and see where he ranks now for a particular keyword, so I have some basis of measurement for my efforts. I normally have my Google Preferences set to 100 results per page, just because I don't like to keep clicking. However, I generally set up my clients to 20 results per page, so they don't get confused (10 is just too few)

My client's page is already up and indexed, without any optimization. If I set my preferences to 20 results per page, his page cannot be found in 1000 results, page click after page click. (Actually I get to about 881 before the filter kicks in)

If I set my page to 100 results per page, he comes in at #517.

Why would the results differ depending on the number of listings on the page?