I know because I've been there. I was underperforming and I'm also naturally very self-critical. So in reviews etc I'd be very open about my weaknesses and the need to improve.
In corporate culture there is nothing to gain from self-awareness. You've just handed your manager an invitation to use you as a scapegoat (see @czep's comment), to leave you out of pay rises and give you crappy tasks.
There's everything to gain from being permanently confident and upbeat about your abilities. Then your manager may A) be fooled by the act and think you're a great performer or B) figure out that you're struggling - but then you're in no worse a position than you would've been by being downbeat in the first place.