Level is a big factor in your total defenses, but the character's class determines the starting point of each defense stat (which can be further modified by attributes, spells, abilities, talents, and equipment). For example, fighters start with the highest Deflection score and they maintain that advantage as they level up. If a fighter really wants to focus on holding a line in melee over doing damage, he or she can equip a shield and gain an even larger Deflection bonus. Unless you're higher level than the fighter, it's very unlikely that your Deflection-based attacks will come close to his or her Deflection defense, meaning you'll wind up missing a lot more than 5% of the time -- and it will probably be impossible to crit them. If you want to hurt fighters, use attacks that target Reflexes or Psyche, which are their weakest base defenses.
Most classes have at least one ability or spell that shifts the defense they are targeting with standard attacks. E.g. barbarians have a Brute Force ability that allows them to temporarily switch over to targeting Fortitude. Against our sample fighter, that would have the two advantages of ignoring the target's high base Deflection and ignoring the bonus provided by the shield. As another example, druids have a spell called Firebrand that creates a weapon made out of pure fire. In addition to doing only fire damage, it targets the Reflexes defense.