Area rows on the Page Analysis table that begin with "Top X" contain the average statistics for the top "X" number of pages for a given keyword search. For these lines to appear, you must enter how many pages you wish to analyze on the second tab of the Page Critic window. The Critic will only be able to do this if you ran a Reporter mission earlier for the same keyword you're now trying to analyze. That's because the reporter retrieves and stores the URLs for the top ranking pages so the Critic will know which pages to analyze.
The advantage to this option is the scores that do well on one keyword or phrase may not do well on another keyword that is more competitive. For highly competitive keywords, it pays to not only try using "top averages" to gain an upper hand, but also averages for the specific keyword you're working on. In this way you can improve your odds of including exactly the elements that a search engine is looking for.
Warning about averages: Averages can be very helpful in many cases to determine trends. Generally you'll want to try and make your own page statistics come close to the averages for top ranking pages, or identify which averages are "significant." However, they can also be deceiving in other cases. For example, if a search engine scored pages highly that either had the keyword at the beginning of the title, OR at the end of the title, then we might report an average keyword prominence of 50%. However, in reality, most of the top ranking pages may have had 100% or 0% prominence scores. Putting your keyword in the middle of the title to match the "average" in this scenario would not help you.
Another potential pitfall of blindly following averages rests in the fact that there are MANY variables that a search engine ranks a page on, so you should be cautious before drawing any conclusions. For example, you might assume that since a majority of the pages on average had one or more keywords in their meta keyword tag, that the search engine supports keyword tags. However, since many Web masters use meta keyword tags regardless of whether the search engine supports them, this conclusion would be wrong for about half the major search engines that declare they do not support that tag.