Why are some New Contributors placed into the Low Quality Posts queue, instead of the First Posts queue?

Question

With some frequency, I see posts that I would say could have been in either queue. Is it decided

1. somewhat randomly?
2. with one of the queues in priority?
3. by some maybe indirect trigger?
4. in a complicated way?

By 3, I'm thinking of, say, a person who is not a New Contributor, deletes their posts, therefore becoming a New Contributor again (as explained in this answer); yet the system knows not to place them in the First Posts queue. I would hope it's like this, because I tend to be more lenient in my voting/comments for posts in the First Posts queue, and I don't really want to have my words fall on ears proven slightly deaf.

For 2, I think I've not actually seen actual spam in the First Posts queue (but could be mistaken).

Examples can be provided (look for closed* posts in the First Posts queue, and the New Contributor notice in the Low Quality Posts queue), but I think people who have reviewed both queues would agree, and it's not really nice to link to people who aren't at their best.

*yes, I know not all closed posts are low quality.

Answer 1

The rules are quite simple:

1. If it's the first post of a new user, it will end up in the First Posts review queue
   • But you are right in thinking that deleting all older posts can make a newer one the First Post, and it will get reviewed too
2. If a post is flagged as Very Low Quality or (if it's an answer) as Not An Answer, it will end up in the Low Quality Posts review queue
   • Flags may be cast by users or by automatic checks (e.g. one-sentence answers are usually automatically flagged)
   • Though I recently encountered some cases where a Low Quality post wasn't flagged

However, if you review a post in the First Posts queue and flag it, you won't be able to review it in the Low Quality Posts review queue (that would be 'validating your own flag', so to speak) and IIRC vice versa there are also some limits. That explains why you usually don't see posts in both queues. Also, the Low Quality Posts review queue is populated with a delay of 15 minutes after the flag.

There's a wonderful flowchart here on Meta Stack Exchange but it only pertains to questions, not answers. Still, it gives you an idea about the complexity of the system when it comes to new users, review queues and flags.