In the chat filter's there are two different options which will show EXP gain.

One is , Own Progression Messages.
(You gain 2000 exp as gladiator)

second is, loot messages.
(You are awarded 2000 exp for objective completion!)

The problem , is that when you have both of these on, since you want to see when you win loot in party, and the things you are progressing on ( ie; when you level up) you want them both on.

Having them both on , when you complete say the roulette dungeon makes it look like this

You are awarded 2000 exp as gladiator for completing a dungeon!
You gain 2000 exp as gladiator.
You are awarded 3000 exp for swift completion of first time objectives.
you gain 3000 exp as gladiator.

You only get 5k exp stated above, not 10. But, 10 would make sense to a new comer or person who hasnt looked this far into it.