i don't know....was he given a direct order "do not tell harry potter!" there are ways to slip through orders and do something else if the person who gives the order is not that smart (i was thinking again about asimov's books where there were few people who could actually master a robot...it was a whole science to know how to give orders so you can get an exact result. but i think i'm alone in talking about asimov's books...has anyone actually read these books?) it's like when you don't lie but you don't mention some facts.
and think about kreacher...how many orders did harry had to give him to make sure it was going to get right (when he ordered him to chase malfoy)