There's inevitably a lot of randomness, given the quantity of things that get posted here. That is probably enough to explain what you're seeing. Flags are mostly by users; moderators don't see most of what gets posted here. We try to review all the flags but that doesn't help if a post doesn't get flagged in the first place.

If you see a post that ought to have been moderated but hasn't been, the likeliest explanation is that we didn't see it. If it's something egregious, you can help by flagging it and/or letting us know at hn@ycombinator.com.

In any case, there's no consistency of moderation across all the posts, nor can there be. But certainly there's consistency of intent: https://news.ycombinator.com/newsguidelines.html