Obviously each person holds a meaning and obviously we somehow manage to understand each other. Yes, there is occasion for confusion, but that's what semantics is for. No clue about why it should be different in large groups (such as a culture or society). And "consensus achieved" is convention achieved: you've misnamed it.
Okay, first off, billions of people do not communicate simultaneously. They usually take turns at talking, and its rarely billions of them trying to communicate at once, and even more rarely with the same word, especially considering the number of languages there are. And secondly, I knew what you meant to say, although it was funny.
People do not fail to form their OWN relationships with the world, and that includes the linguistic relationship. We have cultural, familial and social vernaculars, but we also have unique, individual vernaculars. No two people will approach understanding a word with the same background, the same mentality formed of memory, the same history of experiences, the same social environment, and the same attitude. One person can even approach understanding a word from different places in their single lifetime.
Under "my logic," words are only meaningful. Words cannot lose any meaning, because all they are is meaning, and that meaning is whatever I intend to say. It's all about what I intend to say, and the words I choose to say it with are secondary. In learning the language proper we employ the logic by which the very words came about. It's not always logical, because it has it's own logic born of convention--that is, how people used the word, and how it evolved in use.