Yea I never understood this either. Sure if one guy is getting his ass handed to him and isn't landing much all fight...then out of no where he lands a KO punch...that could be considered sort of lucky...as he was outmatched all fight.
But like you said, fighters intend to put their fist or foot to chin. If they pull off what they are TRYING to do, how is it luck? Never understood that. I can't recall a semi-recent fight ending in luck or something that can be considered luck.
Luck isn't measured by how determined you are to make some thing happen, it's measured by how likely and how often you are able to do that one thing.
For example, I'm a terrible darts player. If I went into a darts game and my intention was to hit the bullseye with my first shot and I actually hit it, that would be considered an incredibly lucky shot. It doesn't matter if it was my intention to hit the bullseye and I hit it, what matters is the likely-hood of me hitting that bullseye consistently. The chances are very slim, because I'm rubbish.
Matt Serra KO'ing GSP could be considered an example of a lucky finish. Here's a guy with no proven real KO power who comes into the fight as a huge underdog and KO's George in the first round. Sure, he intended to KO George, but how often is he likely to pull that off? Not very likely at all.