I think it is normal for a lot (maybe even most?) DMs to fudge entire NPC stats as they game. But I think it requires a certain skill to be able to do it and still be fair to the players.
I personally don't fudge stats unless I can't avoid it. I over prepare.
A reason I began doing this was because I had bad experiences in 2e with my DM fudging NPC stats all the time. As I became more familiar with playing D&D, I realized that every single NPC I encountered, no matter what kind of NPC he was, was as powerful as I was or more, and was prepared for anything that I tried to do to defeat him.
For example, when I reached 9th level Thief/8th level Fighter in Darksun, I thought I was finally pretty tough. When 4 thugs in the street randomly tried to mug me, I thought to myself, "Oh no, not this time." I was going to teach these guys a lesson, and when they initiated combat, it took 1 round until I was forced to run away. I was the guy with a metal sword and a metal dagger and these were "common" thugs. I thought maybe they just hit hard but had low hitpoints, so I stopped and threw my metal dagger at one that I already wounded. The dagger simply hit him, he pulled it out, said, "Ahh a METAL dagger!" And they ran away with it. Common thugs were just as hard for me at that level as they were when I was level 3. I always got a butt whooping. He never referenced books for stats with these random encounters. I'm sure he didn't even have notes with stats. He just knew what our attacks and Thaco was and created a number for his NPCs to be close or better than that.
I also realized that every time I encountered a magic user, he had just the right spell prepared to counter any of our attacks. I am sure most DMs do this. They don't prepare spell lists because it is a pain in the butt. So they use spells that seem appropriate for the situation. I can't stand that as a player. I would love to spontaneously cast any spell from the PHB as I need it. Oh, this NPC has resistance to fire? Ok, I will cast only cold spells. I was bull rushed off a bridge? No worries, I'll just use Feather Fall even though I would have had no reason to prepare it for the day. No spell slot goes to waste when a DM spontaneously casts NPC spells! But mine sure do.
If you can balance it so that I can't tell that you just happened to be ready for a specific attack, then it is not a problem. But if I constantly feel as if everything I do is pointless, then why bother playing? You would obviously have won D&D.