You know how when a cat trips or runs into something, it gives off this look of wounded dignity that says, “I meant to do that all along”? This is an important principle when handling combat in a tabletop role-playing game. Don’t worry what your character (or NPC) intended to do when you rolled the dice! When describing the result of a roll, act as though that’s what you intended all along.
Did you miss that club-footed kobold for four attacks in a row? Are you really such an inept fighter? No, you were just toying with him. Really!
How does she keep hitting you? You’re in plate armor and have a 17 Dexterity! Could it be that she recognizes your fighting style, perhaps from training under the same swordmaster that you did? Then again, it could simply be that the cobra bite that you thought you shrugged off earlier is still slowing your reflexes.
So you were trying to take that bandit alive, but you punched him too hard and now he’s dead. Sure, maybe you just don’t know your own strength, but it could also be that knowing smirk on his face that dared you to do it. He must have wanted to die. Why? What secret do the bandits hold that’s worth dying for?
The game’s fiction need not be wholly defined in advance. Adding things retroactively can be a good thing. Writers and storytellers do it all the time, so why not do the same in your game?