My theory of these things, is that we expect him to miss the grappling hook the first time. In every other movie with a similar scene, the star misses the first time. So we "fill in" that part in our minds.

I saw SW five times in the theater when it was released, in 3 different theaters. I swear that Vader didn't get away... I was shocked when I saw a video years later and he got away. I saw Luke miss the grappling hook. I saw them looking up from Tatooine at the battle (later I learned that this "memory" was the result of owning a SW trading card with that shot on it). Although I now have come to doubt these "memories", I'm still not totally convinced there isn't a missed grappling hook somewhere, possibly on a very early trailer. And I'm not totally convinced Vader's survival wasn't added sometime after the first month, when SW's huge success was making a sequel essential.