Folder shares : parent vs child shares

Recently I noticed that folder shares at the top of the folder structure seem to have priority over shares for the same user on sub folders. Example : the user has a read-only share for the whole folder structure plus read/write/delete rights on a subfolder. Under testing we observed that the user cannot write or delete in the subfolder. So apparently the parent folder rights override the child folder rights. Could not find anything about this in the documentation. Does anyone have a clear explanation of this point ?