A dispel magic removes magical effects upon anyone within the area. This includes effects given from spells, potions and certain magical items such as wands. It does not, however, affect enchanted magical items or spell protections such as Spell Turning, and Spell Deflection. The chance of the dispel succeeding is determined by the level of the caster and the level of the magic being dispelled. The base chance of successfully dispelling is 50%. For every level that the caster of the dispel magic is above the original caster, his chance of success increases by 5%. For every level that the caster of dispel magic is below the original caster, his chance of success decreases by 10%. However, despite the difference in levels, there is always at least a 1% chance of success or failure. Thus, if a caster is 10 levels higher than the magic he is trying to dispel, there is only a 1% chance of failure. Similarly if the caster is 4 levels lower than the magic he is trying to dispel, there is only a 10% chance of success. Intuitively, this spell is almost useless if the target is 5 or more levels higher than the caster. Note: while this spell dispels the individual effects of grease, web, stinking cloud and other such spells, it does not dispel the area of effect.
They claim that although the spell will work as described when a higher level caster attempts a dispel against a lower level caster effect, the actual probability of a lower level caster affecting any higher level caster effect with dispel magic is either zero or close to zero.
I have no mod skills, so I decided to do some front end testing.
I tested BG2 NPC's of caster level 12 and level 13. 13 vs. 12 dispeled effects every time. 12 vs. 13 was not able to dispel any effects after many different castings of dispel magic. I used Stone Skin in my testing, as I know that some other effects (like mirror images and anything in the enchanted weapon slot) are automatically dispelled.
This preliminary test suggests that Dispel Magic works at 100% when the caster is higher level, and 0% when lower level than the effect.
Can anybody confirm whether this behavior is:
A) an actual bug in Dispel (and presumably Remove) Magic?
B) not a bug because my testing is flawed?
C) not a bug because it is working as intended?
D) not a bug because my result was just unlucky?