They're not counting all incidents, they're counting "Part One" crimes in the FBI's Uniform Crime Reporting system. IIRC, that includes murder, rape, robbery, aggravated assault, burglary, larceny, and auto theft. As noted above, that includes some violent and some non-violent crimes.
The ratings are also usually calculated by number of crimes per 1,000 population, so an unusual spurt in one area in a small community could result in a huge increase in their crime stats. Because of their populations, you'll rarely see NYC, Chicago, Houston, LA, Philadelphia at the top of the list, but they might be more risky to visit than St. Louis, which came in worst this year.
You really have to look at the detail of the stats to see what might concern a visitor. For example, I'd be much more concerned about the robbery rate in a community I am visiting than the murder rate, because as a visitor I'm much more likely to be a robbery victim. And I wouldn't be much concerned at all with the burglary rate as a visitor, but I sure would if I lived there.
Also, we have to also remember that if you're at WDW, you are a LONG way from the incorporated city of Orlando. Even MCO is quite a ways from downtown.