Lanefan
Victoria Rules
AC is almost always trivially easy to explain in-world, and levels are easy enough as well particularly if characters have to train into them.D&D is one of the worst possible TTRPGs if you want good in-world explanations for how the rules work. It's horrendous for that, and it's been horrendous since the very beginning. AC, hit points, levels, and many saving throws are some of the worst offenders, and they've been at the core of the game since its start.
Hit points are very abstract and always have been - a necessary evil, says I.
Saving throws are (relatively) much more in-world explainable in the older editions where for example your clothing, gear, etc. could and did burn if you blew your fireball save; the newer version you reference below is indeed somewhat nonsensical.
There are other fantasy TTRPGs whose rules are not as gamey and abstract where using those rules for more tangible in-world explanations makes more sense and you don't quite turn the fictional world into a silly goofy mess like what happens with D&D (all editions) when trying to use the rules as more like physics.
And that ties into my original argument: you're so used to AC and hit points and Reflex/Dex saves/breath weapon saves/spell saves (empty room, fireball, no damage to your clothing/armor = goofy video game logic) that you don't mind them anymore, but if the game changes, yet retains the same gamey logic for its rules, it becomes an issue all of a sudden. It's been that way from the start. Don't play D&D if you want rules that make sense in the world!