I don't think there's anything such as a lucky shot. Fists are thrown with the intention of hitting someone's chin. If you can finish your opponent it sends a clear message.
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.