Some of the popular EXP mobs have been coded to respawn faster if the party that's killing them is of an appropriate level and kills them swiftly. This enables a party to chain mobs to increase their EXP/hour to a much higher rate. Once the party reaches a level where the mobs are no longer an appropriate challenge, they will begin respawning at their normal rate.
Some of these mobs are even in dungeons, which results in them spawning at a fraction of their normal rate!
The intent of this custom change is to encourage and reward skillfully party play between adventurers.