Sometimes I can tell when my current DM fudges a roll to miss an attack or reduce damage. He has a tell in the specific way he pauses and breathes before announcing the roll, then tries to hurry to the next turn, which only seems to happen when someone is in a life-or-death scenario, but “luckily” survives.
Should I let him know he has a tell? Will it be less fun (or more stressful) for him if he knows I know?
…and sometimes that has to happen on the fly.
Sometimes I’ve made a fight too easy so I need to provide the goblins with some backup.
Sometimes the backup was always planned after round 2 but maybe it’s a little less than I planned because they’re already near death. Maybe it’s a little more because they’ve killed most already. Maybe the party was stealthy so less backup. Maybe the party was overly loud so there is more.
Also sometimes the pause to “save” a character comes from determining/calculating if the action is “fair”. Why did they just take that “stupid” action. Maybe I undersold just how powerful this NPC was. But maybe I did describe them as having an otherworldly glowing set of armor and you watched them wrestle a bar full of orcs and slice off one their heads in a single blow.
Perhaps you’ve reached a point between player and DM where this isn’t necessary anymore, but I don’t fault anyone who might fudge a roll to keep the game fun.
I just personally try to balance things without ever taking away from the results on the dice themselves. The changes of plan you listed are all fair game in my mind for balancing encounters. There’s nothing wrong with balancing on the fly, I just think if you’re at the point of lying about the results of individual die rolls, at that point you’re disrespecting both the players and the game system. There’s risk embedded in the system by design. If you don’t want that, play a different system where you can handwave risk away and railroad events without cheating.
That said, this is all just my personal philosophy about GMing and games in general.