I belive it is any bow, Not to sure though

 

This has to do with the way bukkit/minecraft handles deaths. Because it's a bow, meaning a projectile death, it often doesn't know who the killer is. This happens with every bow, but a little bit more often with bows with damage enhancements (like miq bow)

 

The fire doesn't work, if you get killed by fire then that's that. It doesn't say who killed you.

 

No, it happens to everyone. (although I do get the kill often enough)

If you really want the kills, go in close combat.

 

I know there is a way to fix this. I've seen it fixed on tons of other servers.
Although, I don't know how exactly it is done, because I'm never the one coding.