Some monsters actually have negative resistances to certain elemental magic...like IIRC Imps are vulnerable to lightning, etc. and there are many other monsters with negative resistances.
It makes sense that monsters have a main weakness, like casting fire on a sand demon probably shouldn't do anything, while freeze would work well <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> This is just an example, I don't know if this one is true or not.
