Failure to disaggregate appropriately…
The Felony Murder Rule is a law where a person who is involved in a felony can be charged with murder if someone dies as a result of the felony.
The felony murder rule is often applied when an armed victim kills one of the criminal suspects attempting to victimize them. If there is a surviving accomplice, the accomplice may be charged under the felony murder rule.
If someone is charged with murder under the felony murder rule, the homicide will be coded as a murder in the FBI Uniform Crime Reports, not as a justified homicide. Examples of this practice are easily found online.
The effect of this is to aggregate homicides committed in self-defense with murders. This under-estimates the number of justifiable homicides recorded to about a fifth of its real value, which dilutes the number of cases where a lawful gun owner stopped a crime.