"How is it possible?"

It's simple. If the vote count was the only thing that mattered, then you'd have 1000 bots upvoting certain posts to gain money. This vote-count gaming is dealt with by votes backed by money. The more steem power a user has invested, the more $$$ his vote is worth. In this way bots and fake multiple upvoting is defeated. Is it the fairest voting system ever invented? No, but it does solve fake vote issue.