There is no real pathern.
They try to balance it during items implementation on Test Server, by taking into consideration players feedback, besides their own thought about the item, I'm always part of the discussions on test server and my thoughts are:
- Level Range: Items destined to higher levels should be stronger, therefore have more imbuement slots. But that alone doesn't mean much, there are many factors taking into consideration at the same time.
- Item Progression: If a new item is implemented with many imbuement slots, it can throw his predecessor into the trash. For an example, if Cobra Sword had 3 imbuement slots, instead of 2, the Blade of Destruction would be completely useless and forgotten.
- Over Powerness: They don't want an item to be extremely overpowered in any circustance. For an example, if Falcon Plate had 3 imbuement slots, instead of 2 it would be the best armor ANYWHERE. But given that it has only 2 slots, some other armors can be better at some places, for an example: Elven Mail is better than the Falcon Plate at Energy Library. CIPSOFT likes the need of diversity and tbh I agree with it.
These is what I can see when discussing item balancing on Test Server.