This was once much more common, due to the cumulative XP penalties on death. Someone could be lvl 25, and have the exp they did at lvl 20. Now however you can only loose about one and a half or two lvls worth of exp max. So you might be fighting a lvl 20 elf pally with the exp of a 19 elf pally, while your close to lvling from 16 to 17. Just enough for them to drop down into your range.

Sorry if this comes off as rambling, I tend to do that.