The New York Times (on-line edition) reports "...for instance, if you wanted to see which flights between Atlanta and Newark are most prone to problems, you would go to its Flight Rating section and find that while Continental Flight 1154 is on time 96 percent of the time, Continental Flight 1156 is on time only 44 percent of the time and was canceled or diverted 6 percent of the time over the last 60 days." There is no explanation offered.
The article is about web sites that follow fare trends and on-time performance. This item is in the part about flightstats.com.