In a fight, 2 forces go against each other, the stronger force becomes the circumstance or fortune that imposes itself on the other actor. When we say someone got lucky, we say it precisely because they were up against a force, which should have, negated their will.
My definition was lazy and was from Wiki.
Thing here is no one can measure the "force" of a fighter.
Some people will say Erick Silva has more KO power than Paul Daley. No one can measure it really.
Some people would say GSP has a weak chin and wilts when he gets hit. Some people would say Serra, although rarely finishes by KO (he is a Jitz guy with short arms) generates a lot of power because he is physically strong and throws power punches most often over jabs and point punches.
It is all opinion as to which force is stronger in MMA. It is all based on opponents faced. Shogun has very few sub wins. Does that mean if he sub'd a solid ground fighter it would be luck? I don't think so.
There is no science to this. Just loads of practice. Years of it. To do exactly what Serra did. I myself can't count that as luck.