SN uses a model based on WoT to determine what is good content and thus the ranking on the front page. It's not only based on amount of zaps but also on the trust of the users who zap. See https://stacker.news/faq#do-zaps-help-content-rank-higher

But @k00b can probably explain it better

We will create a SN dev blog post about this sooner than later since this is probably the least understood but still very important feature of SN