As Fiberfar pointed out, each class has a certain type of enemy that is easier to beat than others. Fighters are the best in hand to hand combat, hence, melee opponents are easier to beat for them, and garner less experience. A wizard who can survive a horde of melee opponents just dealt with a tough encounter, and gets more experience for it however.
[color="Pink"]"You can do whatever you want to me."[/color]
[color="DimGray"]"Oh, so I can crate you and hide you in the warehouse at the end of Raiders?"
[color="Pink"]"So funny, kiss me funny boy!" / [color="DimGray"]*Sprays mace* " I know, I know, bad for the ozone"[/color][/color][/color]