The internet, in some ways, is just like real life.You'll notice people who carry themselves like they're doing you a favor to be in the same room as them, when they're not really all that attractive.Sometimes you'll meet someone who is very attractive, but has self-esteem issues.
Not to sound sexist, as i believe this usually only works with men, but what a man lacks in looks, he can usually make up for in confidence. If you believe you're a catch and carry yourself like you're attractive and that no one is "out of your league" then you're more likely to fit in with like minded people.
So yeah, there's a chance that someone who believes they're attractive is actually pretty damn ugly in your opinion, but to them, they're the best thing since slice bread.