Remember that EQ was ripped from D&D, which would have been in 2nd ed during that time (
source). Multiclassing in 2ED&D meant splitting your exp among your classes, so you leveled up slower than if you had only a single class (
source). Since EQ only had a single EXP pool, Verant implemented it through a penalty that required more experience to achieve the same level. Then, they made the "pie" system for distributing exp, so hybrids munched more of the pie, "decreasing" everyone else's individual slices.
In Velious, they finally corrected it, but because they didn't want to change current exp totals, they slapped on an equivalent buff to hybrids. I haven't tracked down exactly how the math works out with their fix, but its something like hybrids an additional 40% to level, but their piece of the pie is multiplied by 1.4 once its divided up to them. I think the hybrid penalty removal also stopped their group members from having smaller pieces, but again, I'm not sure how that math was handled.