What am I talking about? Well, stuff like AC for example. AC works fine when it's obtained from dexterity or class bonuses (because then it simulates agility/dodging ability), but mostly AC is due to wearing better and heavier armor. Obviously wearing heavier armor would make a character less mobile and more likely to be hit, but in AD&D it helps them avoid damage. On the other hand, wearing heavier armor should lower damage received when the enemy's blow does connect, but AC doesn't do that at all. So it's completely counter-intuitive to logic and real life.
A lot of these are caused by abstraction. Abstraction is needed for P&P, you won't fucking roll a dice 10 times and then start calculating momentum and shit to decide if your blow pierced through armour or not.
Which obviously brings the question why do game devs use P&P rulesets for video games? So the rules are understandable for somebody who doesn't have proper education, that's why. There's a lot you can write about it(I'd say that complicated, but logical rulesets kill min-maxing better than any other mean but who cares) but to keep it on topic - it works like that because it's complicated enough to simulate armour and avoidance and simple enough to not waste all your day on math.