Every thing is based on chance, RNG.
The rate listed in the ability, 80% stun, is before looking at tenacity and potency
If RNG roll processes the stun. Than potency and tenacity is compared and in some magical formula with rng it is determined if it is applied or resisted.
More potency better chance of applying
I have Teebo Potency at 86% and Rancor resists less but still resists.
More tenacity better chance of resisting
I have my Sun Fac Tenacity at 82% and RG stun is resisted more than it is applied.
Potency and Tenacity is not really "wasted" the higher the number the better chance to apply or resist.