Obviously when we look at a report, if it's a post by someone with an established posting history/someone we at MNHQ know of, then our knowledge of them (good, bad or indifferent) may play into our decision.
I said this last week and everyone told me I was being ridiculous. But IMO and IME it's unavoidable to think "Hmm, Horry has a habit of being a bit goady, better delete this" or "Weird, Horry doesn't turn up in our Reports often, the person reporting is probably being a bit oversensitive".
There are only two ways to make sure every single post meets guidelines: every single post is moderated before it's published, or every single post is read by a moderator within a reasonable timescale. The resources required to do that are simply hilarious.
Now, I've modded on a site that was at the time very similar to AIBU - a mixture of intelligent discussion, foul PAs and derailing. There were plenty of times when the mods between us agreed that "Closing for review" was the only sensible thing to give everyone a chance to cool down. Whether we reopened or not was as often as not dependent on the general mood of the board, rather than the specifics of the thread itself.
At peak we had I think nine mods dealing with around a million unique views a month - so smaller in scale than MN but more mods per view. Theoretically we would read everything between us; realistically you can see when you log on that something has attracted 200 posts in half an hour and will need a squint sooner than a twenty-post "Which pram?" post.
There must be a middle ground between reading every post and only reading reported posts/threads. I would hope that increasing the modding capacity (either staff or volunteer antipodean Nightwatchpeeps) would find that middle ground.