Have you ever compared results in Google when searching nearly simultaneously with Internet Explorer and with Firefox? I recently did a search on “baltimore aquarium hours.” The 6th listing in the results using Firefox was nowhere to be found in the results in Internet Explorer. Very odd.
Today I did the same search again and the results for the first 20 were identical. The mystery 6th listing was now present in both.
In addition, note that results will vary depending on how many results you look at. If you look at 100 results in Google, you’ll see a lot more indentations. This indicates Google is showing 2 results per domain, not just 1. In the case of “Baltimore Aquarium Hours”, when viewing 100 at a time, there were three additional listings in the top 10 that were second listings of a domain. The quality of these links in my opinion was somewhat lower than the sites that got bumped down the standings.
There are some lessons here:

• results may change from day to day
• you may see different results in different browsers – but is it because of the browser or something else?
• sites with top positions do depend on how many sites you’re seeing at a time