http://everquest.allakhazam.com/edit...rs_letter.html
The short of it is that originally, the AMOUNT of experience you HAD determined how much you TOOK. Since a Hybrid needed more to level, an equal level Hybrid HAD more than someone else and hence they TOOK more.
The bandaid patch for removing this was to create free experience when a you killed a creature equal to the Hybrid penalty. They also changed how experience split was calculated to be based on level instead of total experience obtained. This essentially eliminated the Hybrid penalty while at the same time preventing people from suddenly jumping in levels (which is what would have happened if they changed the exp table).
This is why I am a huge advocate for just eliminating the penalty from the start of they create a Re99 server. While it may not be "classic," it would have pretty much no impact on the feel of the classic server while eliminating the need to do goofy gimmicks several expansions later to ultimately remove it anyway.