It is confusing to get at first, but it's actually down to how much steem power the users overall have that are voting in the other topics. So you could have 1000 potentially upvoting but because they haven't bought any steem power the authors get barely any payout.

Now I thought about this, it might seem 'unfair' at first, but it's actually a good way of weeding out any chance of brigaders or bot voting. At the very least. People trying to con the system would have to spend money on it if they wanted to upvote specific users for no reason and just get all the steem for themselves. It also means that people who don't contribute anything and just want to spam won't get payouts either because in theory when you think about it people who actually have something invested in the network won't want to fuck around.