After logging out all of my characters and restarting my server, I still saw the same inconsistent experience statistics being reported. As a result, I decided to perform the same test with an additional 6 spawns to gather more data:
As you can see, the problem continues to happen... but I did notice something...
Commando = War
Blackjack = Rogue
Medic = Cleric
Stonehenge = Druid
Funkmaster = Bard
Wesley = Ranger
It never happens to Medic (Cleric) or Stonehenge (Druid), so my current theory is that the problem is affecting melee classes only.
More testing needs to be done to prove this, but I wanted to bounce the idea off the community to see what you all think as well.