Gunfire often causes a panic, and panicky people produce their own injuries and deaths, notably by trampling or accidentally pushing someone off a ledge or into traffic.

Killed is for homicides. Death includes secondary and tertiary effects.

Edit to add example: whenever someone says “gun deaths” the number they’re about to give you includes suicides, accidents, etc., which outnumber murders more than 2:1

In the context of a shooting I'm not sure there is one. Seems most likely that their graphic design intern wasn't paying much attention when they made that graphic.

Huh. Interesting observation

Maybe killed on the scene or died later from injuries?

Well, one you can still rez and the other has to corpse walk back to get their gear.