Except for when a thick desert robes with some sheet metal body armor stops the round entirely.
Remember, those reports are not coming from the "AK guys" on perfectunion... They are coming from soldiers in Afghanistan.
Edit too add:
Has there ever been a recorded kill shot at 900m with 5.56? That is the record kill shot distance for 7.62x51. It would be a pretty seriously impressive shot from a 5.56.