Because your character notices exploitable gaps in the minions defenses that it doesn't see on other mobs. As you say, a minion doesn't know it's a minion. It's just a minion to the players characters. In other words, for it to be a minion you have to know where to defeat it in one attack, and since you know you can take it down in one attack you know it's a minion.
I could say to the players: "It moves in such a way that you you think you can take it down in one blow", which I often do in a way. For instance: "The crappy skeleton barely holds together, but the claws look kinda frightning."
A bit of circular logic there, but it starts out with the DM deciding that they are indeed minions.
You are basing your description not on the capabilities of the foe, but on your expectations as to the outcome of the encounter.
The creature doesn't have any weaknesses until it is hit. Then, an obvious weakness suddenly appears. You are basing your in game fluff descriptions with what will happen if the PC is successful with an attack. You are not basing your in game fluff descriptions with what will happen if the PC is unsuccessful with an attack. Until the PC is successful, the minion is invincible and could kill the PC.
How exactly was the PC exploiting gaps in the minion's defenses if the minion kicked his butt?
He wasn't.
You are rationalizing specific game mechanics with fluff, fluff that shouldn't exist because the PC should be unaware of the game mechanics.
And then problems start occurring when attempting to rationalize the difference between lower level creatures that are easier to hit with higher level minions that are harder to hit, but easier to kill if hit.
It becomes tougher to rationalize game mechanics descriptions at that point. Do you describe the minions as having weaknesses, or the lower level foes as having weaknesses?
"You think this foe has a great defense, but once you get past the defense, he's a paper tiger. On the other hand, this foe is easier to hit, but can stand up to a lot more punishment." WT??? How does this rationalization make sense?
The point is, the PCs shouldn't really have a clue. They should be pleasantly suprised when they single shot kill the monster "You fell for that old trick? What an idiot! That little Goblin is a tough bugger, but this big Hobgoblin walked right into my dagger.". The PC shouldn't know ahead of time that the Hobgoblin, for whatever reason, is going to die quickly.
Any foe could take a single arrow to the throat (assuming it has a throat) and keel over dead. Until the PCs are actually fighting the foe though and see whether it avoids the single arrow to the throat, they really shouldn't know.
The term minion is not a term the PCs know. It's a game mechanic term that players know. Like AC.
Do you tell the players that the foe with the low AC is easier to hit? Do you tell them the foe with the low Will is more susceptible to illusion magic?
If you don't describe these monster game mechanics to the PCs, why would you describe that the monster has 1 hit point?
It's game mechanics information that the PCs should not have until they actuallly learn it firsthand. Just like AC. The PCs find out how hard the Green Slime is to hit by attacking it, not by looking at it.
Hit points (hp) measure your ability to stand up to punishment, turn deadly strikes into glancing blows, and stay on your feet throughout a battle. Hit points represent more than physical endurance. They represent your character’s skill, luck, and resolve—all the factors that combine to help you stay alive in a combat situation.
How exactly do the PCs know that the foe's skill, luck, resolve, physical endurance, and ability to turn deadly strikes into a glancing blow are super low?
How do they know that the foe is unlucky??? Do you describe every minion as missing an eye or an ear or a finger?
