I must fudge all the time.
I kind of look at not doubling all the dice on a critical or the bad guy happens to have a potion of healing/invisibility or such. Designing encounters is hard and does not take all into account. When the PCs encounter it, I'm not sure I am done making it, since there was no playtest. I'm guessing professional designers have that ability to playtest.
I add more monsters if it feels right, or add HP to the big bad if I think it makes more sense to have him last one more turn. I tend to red circle monsters when they have one hit left on them. It could be smaller creatures with 20hp and the players deal 8-15 per turn. The monster gets a hit for 12 and I just red circle them to prevent me from tracking more things. Next hit kills them, even if it deals 2 points instead of 8.