I think some of them genuinely do think they're ugly. For example I grew up with loads of guy friends...meaning I got "friend zoned" every single time I wanted a relationship. This made me feel very ugly.

Also some of those girls might have thought they were pretty, jealous girls likely started going on about how full of herself she was and made her feel like dirt and rethink how she looked.

Now the biggest part to this issue with self image is air brushing and editing of every model photo. Most really pretty girls notice they have a mark on their skin meaning it isn't as flawless as the Photoshopped to death photos. I've seen an actual model photo shopped to what they want...she didn't look the same at all apart from same hair colour...they even cut off hip size and increased boob size.

As for ugly girls who call themselves pretty. They get haters all the time. They have the confidence to keep going though which personally I find a very attractive trait.

Edit: Most of them really need someone to support them...I mean I felt VERY ugly up until I lost some weight and got my confidence back, this was reinforced when I got my boyfriend. Now I don't feel like I'm anything special but I'm certainly not ugly and if anyone does feel like that it is their own opinion, I don't find everyone pretty either.