Because that is not always the case.
I am not really sure @ what times it happened to me, but i've had that a couple of times that i shoot them, they use /kill, and nothing counts on the server.
in my heart & mind it does count as my kill!!!anyway
I don't really care about if it would count or not on the server, just helping with the suggestion so that those statswhores will cry less & my screen is less infected with bullshit