I have known one person like this in my life. And it's kind of complicated to understand why someone would say they are transgender if they aren't. It's not necessarily to be "fashionable" or "cool". Sometimes it's to fit in with a group of people who you get on with.
This person was... very lonely. Very unhappy. Felt very marginalised and like no one would listen to them. I had several conversations with this person, through which it was revealed that they identified as such because they felt part of a group who didn't judge them. Didn't try and tell them who to be. And were generally warm and encouraging towards them. Which, in their case, was something they'd never really had before. They did it to fit in, and to be a part of something they were looking for... if not in quite the right way.
It took a while for this person to admit they weren't actually transgender. They were probably scared of my reaction, or whatever else. But having come to understand this person and why they behaved how they did, I kinda knew why, and the mental process behind it. Sometimes people just want to fit in, and a place to call home. And sometimes they do things that they may not think all the way through in order to get it.
I understand that need for finding people who don't want to hurt us, who don't want to laugh at us. So I couldn't be angry or upset with this person. Truth be told, trans people are some of the most accepting and understanding people I've ever met. So I can understand why someone like the person I spoke to would pretend in order to fit in and to be a part of that. Even if I don't think it's necessarily a good thing for either them or the people they associate themselves with. Humans are social creatures.
Perhaps there are some people who say they are trans just to be different, or cool, or whatever. But not everyone is like that. And sometimes you just have to understand the individual to understand why they do the things they do.