There are two types of messages, the PM one and the post to the profile one. The post to profile is seen for anyone that can click on our profile.
Those message are not really "private" since they are not done in that section. It's like sending a letter to someone vs just writing it on the side of their house. Anyone that drives by the house can read it, the profile posts are just like that, if someone can click on your profile, they will see it. If you want a message sent to only the recipient, use the PM option which is sending a sealed letter to a specific person.