Why is it I can hide my FaceBook Status …

Why is it I can hide my FaceBook Status Updates from certain ‘friends’. I can hide comments friends make on my wall from other ‘friends’. But I can’t hide comments I make on my friends posts from my co-workers.
I know facebook is probably saying, but if your friends and co-workers are friends than that wouldn’t work, but my friends and co-works would NEVER be friends!