Dispel Magic - Help me understand it

First of all, just to be clear I do have SCS installed, and that's it.

I'm having trouble understanding Dispel Magic... either that, or I do understand it correctly and just got incredibly lucky against Nimbul. I'll explain:

So my level 1 Inquisitor cast Dispel Magic at Nimbul while he had Stoneskin and Mirror Image up. To my disbelief, it was successful at dispelling both! (and I won the fight). The reason I was so surprised is that based on the spell description, I would've had a 1% change of successfully dispelling, because Nimbul is a level 7 mage/thief apparently... 7 caster level minus 2 caster level (Inquisitor is doubled) equals 5... 50% base chance of dispelling minus 50% (5x10%) equals 0%... and hence only a 1% chance to dispel (hopefully that all made sense).

So unless my logic is incorrect, I just got incredibly lucky... or am I missing something?

Thanks for any help/input!