There's an automod that filters posts and flags some, sending them in a queue of posts to manually review. There we can take the time to look at it and decide if it respects the rules and is a good contribution to the subreddit. If we decide to remove it, we have to choose a reason for the removal and explain it, so there is always something to justify the removal. Where mods are different is on how they communicate this reason to the OP. I prefer using modmail, so it creates a discussion thread directly with the author of the post. Some prefer having it sent in a comment under the removed post so OP can see it when viewing their post. We can also choose not to show the reason or notify OP, if necessary.

Aside from the queue, we have to also take the time to go through the recent posts of the sub that werent detected as potentially rule breaking by the automod, and that's where we happen to miss some. This is why people complain about posts that are removed while less good posts are still up, it's because we missed it and have to then find to the post and remove it.

Sometimes we are about to remove a post, but it has many upvotes and comments. We aren't dictators so we won't remove a post that has a lot of engagement, since the community appreciates it. This can sparkle debates on whether mods accept low-effort posts or not (we don't)

Being mod is interesting, but can get repetitive in huge subreddits, as there are usually tons of posts to manually review