It's normal for any plugin reporting your XP/AAXP to not show the values every single time, for every toon. Your client is given (from the server) what your XP is, from 0 to 99%, with 330 'tick's. That's basically right at .303% per tick. So when the server tells you to move a tick (your toon data shows it from server), then it's changed - and any xp plugin will see that changed, and let you know you got 0.03% XP.
It's just like dewey described.
The reason it shows yellow you gained XP on all, is because you did. However, each kill is not enough to increment the character's xp data a 'tick' (1/330), and each character's specific place within that level (again, basically 1/330) isn't going to be exactly the same, so they'll not each increment all at the same time. If you are roughly getting a 'tick' every 2 or 3 kills, then you'll see that tick increment on some on kill X, and others on kill Y, and maybe even others on kill Z.
Hopefully that makes sense. Again, the plugin(s) only report when they see those ticks change (1/330) in your character data, because the exact XP you get for killing a mob does not get sent to your client. If it DID get sent, we would show that instead of just having the client use it's built in message (where it says you got xp from server).
If server said you got XP, you did.
I ain't even gonna read back through my message lol... I probably sounded mixed to you!
Anyway. Same as what dewey said few messages back, just more info. Normal.
htw