The kill tracker can be pretty neat at times especially if you forget how many bosses you've killed. It is annoying though, that you have to kill the bosses to see how many you've killed total. When you kill a boss and you get the experience but not the tracking count - it doesn't count.
I found this out the hard way while doing my last Greater alliance. I had killed every boss and went to turn it in but it wouldn't complete. So I went back and killed one more of basically every boss to see where I went wrong. I ended up killing 6 and 7 of some bosses and finally when the Monkey spirit spawned again (the one that I never got the message "You have killed ** Monkey spirit") it said that he was my fifth one. I'm positive I had killed it 6 times at that point so it's clear without the message you won't get the credit. Pretty strange and frustrating when this happens.