Fair enough.
But here's a different perspective from the other side, having been on the receiving end of a lot of these "Oh I'm going to fail, only did <understated #> hours of study this week blah blah blah, bound to get <very low> score blah blah blah I wish I was smart like <you, someone else> blah blah blah" from people I know (having been in "high achieving" cohorts in school in now in uni), I actually find it quite annoying.
I can see what they're going for, some kind words and some confidence as you said, but I'm not sure they're going about it in the "right way", especially when it happens again, and again, and... again.
Now I personally have a few problems with this.
1) From the outside it appears they're just fishing for compliments and lowering everyone's exceptions for when they pull out a great score. Which really appears to be wasting everyone's time.
2) Over-modesty is actually worse than arrogance for me. I'd honestly rather have a guy say "I'm going to smash this exam" and actually smash it rather than have a guy saying "I'm going to fail this exam" and then smash it.
3) In some regards, shows a total disregard for the feelings of everyone else. Say a high achiever is talking to friends who consistently are "lower achievers" than them (by marks, for example), will those friends feel better by their smarter friend who they look up to saying how "screwed" he/she is for the next assessment? Will they feel any more confident? Probably not, it's likely to lower everyone's confidence. I actually know a few people who do this on purpose, and I really hate it. It's selfish, disgusting and I'm not a fan at all.
4) The people who say all of this, but ALSO then are the people who ask about everyone else's marks (or at least those of the "top students") and are pretty keen on letting everyone else know how they went. It's worse if they ask for someone's marks behind their back too. Ultimate academic douche-baggery in my books.
5) When you do give them some comfort, they say something like "But look at <person X>, they've done soooooo much better than me and done sooooo much more study". I can't understand why some people care so much about other people's marks, so at this point I give this person a blank look and walk off to a more chilled crowd.
6) People using the word "fail" to mean what a friend might consider to be a "good/acceptable score". I know people in uni who do this, and safe to say they are the ones with less friends than all the others.
Not saying you one of those people who when saying "I'm actually not going to go so well" end up saying it like above (in fact I'm certain you're probably not one of those people just from your forum presence!), but I know a lot of people who don't mean to sound like that, but actually do because they've never been on the receiving end. And from observing, those people end up wondering why people don't want to talk to them about academics or why they have less mates than they started off with. In a way, I feel sorry for them for that ignorance.
I have no idea if any of that was coherent, but it's a topic that really frustrates me having been in environments where I'm hearing this stuff nearly ever second day (usually from a minority of people).
/rant, feel feel to disagree.