PoS + Dispel works sometimes?

[mdogornone]

Greetings,

So i was leveling the other day and i had a rogue with much more experience than i (45 necro main) tell me i should be using Pact of Shadow with dispel to heal my teammates and pet whenever i can. I started using this and realized it gets rid of the debuff almost instantly, so it costs (in total) 40 mana and about 41 hp to heal 324 hp. I'm ASSUMING this works the way it is supposed to, and isn't a cheat (since he told me about it).

So now my question: Sometimes when i cast dispel it doesn't get rid of the recourse, and instead gets rid of a random buff (not the first buff, which i make sure is where my recourse is). Why does it work 19/20 times, but 1/20 dispels a random spell. Any help here would be appreciated. At present i figure maybe I'm doing something wrong?