If you listen, Ann Robinson always says "It's time to vote off who YOU THINK is the weakest link ...".
The caps are mine, but the point is, as often happens, the person who is the weakest link is not voted off, because contestants perceive that someone else is worse. The voice over always says whom the weakest link actually is, and often asks if the contestants' choice will reflect this - and sometimes it doesn't.
In the case of everyone answering correctly, it can come down to tactical voting, getting rid of a potentially strong contestant early on in the game.