The way I see it…
In ESB Vader is a total badass, he’s just toying with Luke then when he gets pissed off he starts throwing stuff at him and eventually chose his arm off. Vader has no intention of killing Luke he would have done so easily.

In ROTJ again he’s not really trying to defeat Luke - In a straight fight you’d probably put money on Vader, seasoned Jedi killer… But he’s conflicted and not quite all there. So in one of his evil swings he’s talking to Luke and Luke flips and surprises Vader we get some epic music and now Vader loses an arm.

We don’t see a straight out lightsaber duel until TPM, really.

So in Rey vs Kylo again Kylo isn’t really trying to win he’s trying to make Rey submit. In addition he’s a bit messed up - just killed dad and took a bunch of various hits already. Something stirs in Rey, she lets the force take over or something and takes him by surprise and as he’s not fully focussed she has the upper hand. It’s really not that different to ROTJ, even the music is pretty good at that point, there’s a great line when she’s standing over him.
Ren is flawed, extremely powerful but makes a mistake… Just like Anakin who was in uber power +20XP mode made a mistake with the higher ground 😶