But my reason for the monster running away was mostly about pacing - we’d spent a lot of time in combat and I wanted to change pace. Also I had a sliver of an idea that the monster might now become a recurring villain.
I can in game justify the decision however I like but I’m honest enough to admit that my decision was not driven by any “in game” knowledge.
So, am I fudging now?
I really thought I was fairly clear before, but since you asked again I'll reiterate:
You made the decision based on the narrative of the scene which is exactly what the DM is supposed to do. You didn't establish the parameters of the world (the monster's AC and HP, for instance, along with attack modifiers, damage, saves, and such) and then decide to arbitrarily change those parameters on the fly
simply because the challenge wasn't going how you wanted it to. That would be like a sport's referee changing a call on the field simply because the game wasn't going the way he wanted. The players would not like it and I doubt the fans would, either. (
Sure, your D&D players might like if you ruled in their favor, and if they know
you are doing that and ok with it, then there is no issue.)
A non-number example would be this: a monster's motive might be, clearly, to eliminate the threat to itself (the PCs). Yet, because you simply don't want a TPK and the players are too stubborn to flee, surrender, negotiate, etc. you decide instead to have the "winning" monster retreat to spare the PCs. Clearly, the monster would push on to defeat the PCs, but your meta knowledge of the PCs' condition and likelihood of a TPK made you decide to "spare them".
Now, that isn't necessarily "fudging" (a term more appropriate to numbers and changing die rolls), but it is along the same lines IMO. I play the protagonists as the world dictates (and my imagination of course) they would act. Now, if a monster is toying with the PCs, then retreating and giving them time to recover before confronting them again would make sense---I did this in my 1-20 level campaign, with the BBEG wizard actually used a
wish to raise a defeated PC from death--just to kill him again later on!
I hope that is clearer?