I think there are some cases where reaction fire experience will get awarded simply by "trying" to react. Like, you react, but can't shoot due to no line of fire, or out of range, or out of TU, or another soldier reacts instead, or you can't react because the enemy's melee attack chain is uninterruptable. The point is, while reaction fire exp should logically be awarded only after you actually pull the trigger, it currently is not.