Some sites are penalized because they have been the source of many lightweight submissions. You may not be surprised to learn that reddit.com is one.
However, there are many ways for such penalties to get overridden. For example, if the submitter is known to have posted many fine submissions in the past, the penalty doesn't count. Your account is one of those. Thanks for posting fine submissions to HN!
By the way, it's not hard to get answers to things like this that you're wondering about. You simply need to ask us at hn@ycombinator.com.
A combination of what the community has liked in the past and what moderators, plus a small number of story reviewers, think it might find intellectually interesting.
> it seems that his submissions are not highly upvoted
However, there are many ways for such penalties to get overridden. For example, if the submitter is known to have posted many fine submissions in the past, the penalty doesn't count. Your account is one of those. Thanks for posting fine submissions to HN!
By the way, it's not hard to get answers to things like this that you're wondering about. You simply need to ask us at hn@ycombinator.com.