Even great free throw shooters can fail when conditions change. Jason Kapono shot over 90% his senior year of high school. In his soph, junior and senior seasons at UCLA, he shot 87%. In his NBA career he shot 84%. But in his freshman season for the Bruins he shot only 68%!
How could that happen? He was unable to adapt in his transition from high school to college, even though he clearly was a great free throw shooter. So conditions matter, whether for a whole season or a single game.