Could be due to the fact he already unlocked the trophies & then the game forced it to stay that way. Some games allow users to re-obtain trophies, such as not including the programming needed to prevent it from unlocking more than once (probably just an oversight with the game while it was being made). A lot of games allow users to re-unlock the trophies, such as trophy popping because of this oversight.
I'm testing other saves & seeing if the difficulty trophies are locked completely or not.