[August]

This isn't a mistake - they had to purposefully code it this way.

Example:

Human Druid = 0 modifier
Human Enchanter = -10 modifier

That means that to reach a certain, arbitrary level, if it took 100k experience for the druid, it would take 110K experience for the enchanter. Let's call this level 30.

So, they're both at level 30. Druid has 100k experience, Enchanter has 110k experience.

They kill a mob worth 1k experience total.

Druid gets 100/210 * 1k = 476 experience
Enchanter gest 110/210 * 1k = 523 experience.

Let's say it takes 20K (doesn't matter) to level. That means it would take 22K to level for the enchanter.

Each mob gives 1k experience.

Druid requires = 20,000/476 = 42 mobs to level
Enchanter requires = 22,000/523 = 42 mobs to level

Thus they level at the same time.

This was 100% intentional. It preserves level gaps because the relationships cancel each other out, and all that ends up mattering is the exp modifier, group bonus, and how fast you kill. Everyone levels at the same speed.