>>717651435Luck in fate isn't luck as we normally think of it. It's defined as the ability to defy fate. Cu's spear is fated to always strike the heart, the effect is already decided the moment he uses it, and reality will just twist and warp itself to make that happen. But if one has a high enough luck stat they can defy that fate and potentially dodge the undodgable spear.
Fate makes more sense when you realize the servant system works like a video game. It takes the legends of these heros and translates them into game mechanics or skills. So if someone defied fate in their legend, the grail will realize this as a high luck stat. If someone had a grand last stand, the grail will give them the guts skill. If it doesn't have an appropriate skill for something it'll edit an existing one or create a new skill entirely, and then use that skill in future summonings. So if you think of it like a video game his gae bolg would work like a special move that says "if LCK<B hit rate 100%". Saber had both the necessary luck and enough speed and reaction time to dodge the hit thanks to her instinct skill.