Its because fighting other players is different then fighting npc's. The differences of chances to not hit the target is less for players then monsters. Starting at 62 you should be able to get most of your spells through to your opponent thats level 70 unlike monsters which goes by the color system to where you will have an innate 50% chance of the monster resisting if its 4 levels or higher then you. I cannot find the exact formula but from my reckoning when they redid the resist system for pvp before Burning Crusade came out it was 50% resist or hit/miss rate for players who would con you white. Note that this is not counting the resistances of spell type but the basic innate resist chance. Any spell int he game has a 5% chance to be resisted outright even holy. But having 100 in fire resistance gives you a roughly 20% bonus to outright resisting any spell casted at you that has no penetration along with the 5% innate chance to resist thus why Warlocks are nasty opponents with felhunters if they are over your level. With 100 in fire resist your opponent has a 25% chance to resist your spell outright but has a 40% chance to have a partial resistance that is then based on a pusedo random generation. If everyone truly wants the specific formula I can try to find them again.