Hey Mark,
Our apologies that the results weren't as you expected. Unfortunately, this is something of an intractable problem. The search engines - Google, Yahoo! & Bing - all maintain multiple indices that run across multiple datacenters. This means if you query different datacenters, you can get somewhat different ranking results. Other elements, such as personalization, geography and search history (even if you're logged out of your Google/search engine account) can also influence ranking positions. Hence, there can be a lot of variation in what different people, searching from different computers/locations might see in the rankings.
Our solution has been to run searches from a variety of IP addresses and IP blocks using non-personalized, search history/location agnostic requests. In our experience, these have provided the most accurate results, showing what the "most" users see, though we know there's still quite a bit of room for fluctuation.
Thanks again, and have a great day!
Nick