"the stronger they are the less likely they are to be stunned"

That's a pretty vague statement; "stronger" has to be measured by some in-game value, which would imply that some numeric statistic increases resistance to stun. Other effects (hit, dodge, crit) have different values factored in from both attacker and defender. What does it for stun, if anything? Level, perhaps? Or maybe it's nothing at all and the 50% stun rate manifests itself as streaks of good and bad luck over time.