Has anyone noticed people abusing terms of mental illness?

This is just something I've noticed amongst kids at my high school. I don't know if other people do this in the adult world, but it's something I've strongly noted in my "people"(damn, adolescents and teens can be such brats sometimes).

I would tell them that I am having a hard time doing something because of one of my mental health issues, and then they would say, "Oh, I have that too." or "Yeah, I've was like that before."

I'll give you a few examples:

One girl (a cheerleader that I knew about) who was in my biology class in sophomore year claimed she had ADHD. I saw her sitting perfectly still, wasn't fidgeting, staring straight at the projected screen while going over our lectures, and was raising her hand several times to get answers right.

Later she gets out of class and says, "I wasn't even paying attention."

Question: How were you not paying attention?

You were actively participating throughout our class, and you seemed quite focused when we were taking notes.

Second girl (actually I think she was a woman), very cheery for someone who told me that she goes through depression. Wha-, I mean, how? Do you even know what depression is?

I swear I've met several people already met who say that they have ADHD, depression, or some other disorder.

It's like they take it as a joke. In a way I understand that they are trying to show that they have been through the same thing (even though I'm pretty sure that they're just confused or ignorant), but it really disappoints me when I find out they have nothing in common with me.