Facing an opponent with an AC of 12, you roll a 19 on the die for a total of say 23. It seems you've completely outclassed your foe on that one. And then you roll snake eyes for damage. Oops, turns out you didn't do as well as you thought. Or you could roll an 8 on the die and just barely hit the foe (apparently), but then you roll 2 sixes for damage. Turns out what should have been a glancing blow was really a massive strike.
Similarly, you can't jump into narrating a Reaper attack based on the result of the d20 roll, because there's that damage to account for. If you say you swing over the creature's head, and yet cause damage, you'll look as silly as saying that you *just* got a glancing blow through your foe's defences, but then roll max damage and kill it in one shot.
Quoted for truth.
And this kind of rather dodgy narrative logic between game rules and fluff is all across the game, which is why I just can't get worked up about it. And why I have a hard time seeing another person's viewpoint where one dose of dodgy logic they can gloss over, but another one is the straw that broke the camel's back.