Because he keeps changing his form.
Actually he fukked his back up the last season in miami. Which messed up his jumper and free throw shooting. You can literally look at his free throw percentages and see how it dropped since then.
He even mentioned how his back was all fukked up in 2015 season and playoffs and how he had to turn to military situps, push ups and etc to strengthen his back and core cause his back was so messed up. Thats also why he changed the release of his jumper.