Why aren’t Tenacity and Potency equal?

It’s an idea that’s been rattling around in my head that those two stats, while thematically opposite, don’t function in opposite ways.

This is seen in the two Tenacity effect - Up and Down. Tenacity Up makes debuffs impossible unless specified as irresistible. Yet Tenacity Down doesn’t make debuffs guaranteed. There’s always a 15% (?) chance to resist. 

Let me clarify this isn’t a demand for change, it just seems a little odd.

If there’s always a chance to resist, shouldn’t there always be a chance to land? Or if one makes debuffs impossible, shouldn’t it stand to reason that the other makes debuffs guaranteed?

It’s weird that the effects aren’t fully flipped, but that’s just my opinion.