Well. Typed out an entire post on my phone, and guess I didn't post it. Short version.
I'm guessing that FB, EQ, and TS are similar in that for the spell to succeed a minimum "score" has to be achieved and then once it has succeeded, the effectiveness of the spell is determined, with some randomness thrown in the effectiveness.
Racial defenses and defenses from wiz/schools might not be factoring correctly, or the minimum "score" is just too low.