People report defensive gun uses when they're the offender too. It makes 100% logical sense - people usually think they're justified. Who says "yeah I pulled my gun, but I was the bad guy."?
The data bears this out:
http://injuryprevention.bmj.com/content/6/4/263.full
In our survey, the criminal court judges who rated the incidents determined that at least half were probably illegal—even after assuming that the respondent had a permit to own and carry a gun and described the incident honestly. We expect that the true percentage of reported self defense gun uses that are illegal is higher than 50% for at least two reasons. First, three respondents reported over 58% of the self defense gun uses, and none of their accounts were read by the judges (since all refused to provide a description of the most recent event). Many reported self defense gun uses from a respondent creates a suspicion that the uses may be aggressive rather than defensive.
Second, the reports read by the judges are only one side of a hostile interaction that usually occurred months or years before the survey. We expect respondents will view the hostile encounter from their own perspective; in any mutual combat both participants may believe that the other side is the aggressor and that they themselves are acting in self defense. In addition, when describing the event, respondents will typically want to present themselves in the best possible light.
More:
https://www.hsph.harvard.edu/hicrc/firearms-research/gun-threats-and-self-defense-gun-use-2/
Using data from a national random-digit-dial telephone survey conducted under the direction of the Harvard Injury Control Center, we examined the extent and nature of offensive gun use. We found that firearms are used far more often to frighten and intimidate than they are used in self-defense. All reported cases of criminal gun use, as well as many of the so-called self-defense gun uses, appear to be socially undesirable.
And like I said, gun use does not significantly protect you:
https://www.hsph.harvard.edu/hicrc/firearms-research/gun-threats-and-self-defense-gun-use-2/
- Self-defense gun use is rare and not more effective at preventing injury than other protective actions Victims use guns in less than 1% of contact crimes, and women never use guns to protect themselves against sexual assault (in more than 300 cases). Victims using a gun were no less likely to be injured after taking protective action than victims using other forms of protective action. Compared to other protective actions, the National Crime Victimization Surveys provide little evidence that self-defense gun use is uniquely beneficial in reducing the likelihood of injury or property loss.