Average resistance may be no higher than 75%. Of course, what it takes to reach 75% average resistance depends on the spellcaster's level. One quirk here is that casters below level 20 are treated as if they were level 20 for purposes of calculating the resistance scaling here. So against a spellcaster of level 20 or below, it would take a resistance stat of 100 to get 75% resistance; level 30 would take 150 resistance; level 50 would take 250; level 60 would take 300; and level 63 would take 315.
Why is it called "average resistance"? For spells that have a non-damage effect—such as slow, root, stun—you'll either take the hit or avoid the hit altogether; these are examples of binary spells. The average resistance is the chance you'll totally avoid the hit. Adding the word "average" is a little redundant in this case.