If I start with a weaker opponent I can always fudge it a bit to provide more of a challenge, but when my NPCs end up not doing any damage because the PC in question uses a nice martial technique or combo I didn't anticipate, then that's an acceptable outcome.
But that is part of the appeal for me - the surprises. Sometimes plans and combos should make a fight easy.
Well, I won't fudge monsters except for tactics. If I'm killing people monsters will suddenly make a little bit worse choices, but that's it. Their stats are their stats. I don't want to deprive the players of their joy from defeating a monster fair and square. And they know I run it like that.
But it's not about surprises. It's about straightforward power levels. For instance, I was running an adventure from Dungeon Magazine. There was a Lich on one level of the dungeon. It was a mega-dungeon and the PCs were slowly making their way downward. They had run into books, inscriptions, NPCs, legends and so on that all said "This guy is one of the most powerful wizards on the planet, he has lived for a thousand years and he has had all that time to come up with plans and powerful spells."
The fight was set up to be hard. It had a CR of 4 higher than the average party level. I knew going into this that the party might all die or have to run because of the nasty things the adventure had written for the Lich.
Then, they walked into the room he was located in. They spotted him in the back of the room. The Wizard of the party won init. He then used a spell followed by a quickened spell(can't remember which ones, but I believe they were both Reciprocal Gyre and the Lich had around 10 spells up...he was able to do both due to Metamagic Rods). It did a total of a 150 damage. The Lich had around 145 hitpoints.
It was a surprise, but it made for a bad game. When you are working up to an encounter like that, it should feel hard. Obviously, the goal is always to let the PCs win so the game can continue, but they should feel like they were brought to the brink of death and that they barely pulled it off due to teamwork, strategy, and possibly pure luck. Not simply because they cast 2 spells in the same round.
I, personally, hate when battles end that easily against anything that isn't a nameless guard or easy to kill orc. I'd prefer them to go on 5-6 rounds and feel like I came to the brink of death.