I don't play D&D as a competition between myself and the players, so that definitely skews my perception a little bit.
In my game, the intent is for everyone to have fun, DM and players. It's a cooperative play style.
Absolutely. And if the players feel that the DM is cheating and setting arbitrary numbers designed to thwart their strengths, they may well get offended by that. If a player cleverly and carefully designs his PC to have a to-hit number 4 higher than normal for his level, but then you arbitrarily adjust the ACs of your monsters so that he has the same percentage chance to hit as if he did not make those clever choices, he may well feel slighted that you cheated him out of his choices. Choices he paid for at the expense of other options.
If you set your ACs arbitrarily high and can't justify why they are that high (by knowing the numbers that make up that AC), then the PC wizard who focused on debuffing monsters gets short-changed. The PC who focused on increasing his damage output above the norm is slighted when you jack up HP because they are killing monsters quicker than you like.
Point is, with such a micro-managed system as 3e, a lot of player choice has specific, defined, expected advantages within gameplay that is thwarted if you "cheat" on your monster/NPC creation and just do whatever makes it as hard as you think it should be for the PCs. You are short-changing the choices they made, and the knowledge they've gained and put to use about how the game is expected to work.
It's no different than watching while a player spends three hours creating an awesome Loremaster when you know your DM-style is not to utilize knowledge skills hardly at all, and you never call for PCs to make knowledge rolls.
If the strength of a system is meaningful choices, you undermine that when you make those choices meaningless.
In 4e, what they did is change the balancing trigger from being building everything on the PC framework, to building a set framework for monsters/NPCs, but one that is nonetheless affected by meaningful PC choices. A PC focusing on applying effects to monsters can know and have a reasonable expectation of how that will work, what basic numbers to expect in defenses, etc. If you arbitrarily change the numbers, you undermine their choices. The difference in the editions is that 4e makes it much easier, and much, much less time consuming to stick to the math that is built into the system.