I personally think that either experience or stamina or both have an impact on free throws. I have a player who has screwed me over a few times during close matches. He is supposed to have respectable FT but in at least 2 matches this season, he has thrown up bricks and it lead directly to losses for me.
However, his stamina is not good and neither is his experience.
Finally, I also wonder if there is better success rates when you play at home.
Run of the Mill Canadian Manager