There's also a certain shrewdness to it, albeit the shrewdness a lazy/socially retarded teenager would exercise, and it didn't start within Tumblr. Within circles of kids like this, there exists an unspoken imperative to make these things plausible. Again, see Real Vampire Guy: He wants to believe this, and for people to believe him, so badly that he's reduced the signs of being a Real Vampire to shit anybody can do or perceive themselves as doing. You don't have to burst into flames in the sunlight or gouge out people's necks with your teeth to be a Real Vampire, you just have to have some of these vague qualities that are almost impossible to verify. It lets you be a part of the secret cool Real Vampire club, even if it's only in your head and only as often as you reaffirm it to yourself. For another example, consider the magick/Wicca/Satanist/whatever kids I'm sure most of you knew in reality or observed online back in the day who generalized and handwaved the whole concept into meaninglessness. You know how it goes: "Being a witch doesn't mean casting spells/not going to church/doing anything meaningfully different from how I lived my life before 8th grade."
Now compare, say, the meteoric plummet of any meaning behind the term 'asexual' on Tumblr. It arose as something that, on the outset, seems like it'd take little to no effort to perform. You just don't feel attraction to either gender or want to perform sex acts, and nobody on the internet can really check up on how much sex you're not having. All seems right until you realize you can't keep up the charade when every second post on your blog is either erotic fanfiction or pictures of dudes with you lusting out in the tags. Suddenly keeping up the con would mean a vast overhaul of your hobbies, or at least the public face of your hobbies. Now when someone calls you out, it becomes necessary to explain that it's totally possible to be asexual and feel sexual desire/enjoy porn/write pervy fanfiction with a hand under the desk. When that doesn't hold water, you make up a new thing where you're totally into sex (almost always straight sex, because who the fuck are we kidding) but only with certain people and only once specific emotional and interpersonal prerequisites have been met. This makes you a demisexual, or in any other social circle outside the nerdy end of Tumblr, an average but discerning sex partner. There's an even greasier little movement that argues that one shouldn't have to experience bodily dysphoria to have claim to the label of transsexual/transgender, completely disregarding the fact that its being a legitimate physical problem and not a Feel is what's gradually enabling people to pursue treatment.
What it boils down to is this: They want to be Special, not Different. These are separate things and they know it, at least subconsciously. Being Different is difficult, it's uncomfortable, it's uncontrollable, and it's not generally something you get to define yourself.
